• Father of the periodic table = Dmitri Mendeleev (a Russian chemist)

Mendeleev’s Periodic table

Dmitri Mendeleev arranged the known elements in the increasing order of their atomic weight in the form of the table called Mendeleev’s periodic table.

Mendeleev’s periodic law : Mendeleev’s periodic law states that “the physical and chemical properties of elements are periodic functions of their atomic weights.”

It means that if the elements are arranged in order of increasing atomic weight, the elements with similar properties are repeated after a regular interval.

Features of Mendeleev’s periodic table:

The modified form of Mendeleev’s periodic table consists of:

1. Nine vertical columns- Groups:

There are nine vertical columns numbered as I, II, III, IV, V, VI, VII, VIII and zero (noble gases). Except for groups VIII and zero, each group from I and VII is subdivided into two subgroups A and B.

2. Seven horizontal rows- Periods:

There are seven horizontal rows numbered as 1 to 7. The periods are divided into short periods and long periods. The first three periods are called short periods as they contain fewer elements. Fourth, fifth and sixth periods are long periods.

3. Position of lanthanides and actinides:

The lanthanides (14 elements after Lanthanum) and actinides (14 elements after Actinium) are kept separate in two horizontal rows at the bottom of the periodic table.

Note : Only 63 elements were known when Mendeleev’s periodic table was created. 

Limitations of Mendeleev’s periodic table:

Some main limitations in Mendeleev’s periodic table are as follows:

1. Position of hydrogen: Hydrogen forms both the positive ion like alkali metal (group-IA) and negative ion like halogen (group-VIIA), hence it resembles the elements of group IA and group VIIA. Therefore, the position of hydrogen in the periodic table is controversial.
2. Position of isotopes: Isotopes of the same element have different atomic weight. According to Mendeleev’s periodic law, isotopes of an element should be placed at different places in the periodic table. For example, hydrogen needs three separate positions for protium, deuterium and tritium with atomic weight 1, 2 and 3 respectively. But isotopes were not given a separate place in the periodic table.
3. Position of lanthanides and actinides: Lanthanides and actinides group consists of 14 elements each. All these elements have been grouped in a single place in the III^rd group respectively in the periodic table despite their different atomic masses. They have not been given proper position within the main frame of the table but are placed outside in two separate rows at the bottom of the periodic table.
4. Position of anomalous pairs of elements: Certain elements with higher atomic weight were placed before the elements having lower atomic weight. For example: Ar (at.wt.=39.9) was placed before K (at.wt.=39.1) ; Co (at.wt.=59.9) was placed before Ni (at.wt.=58.6), etc. These pairs of elements do not obey periodic law.
5. Separation of chemically similar elements and grouping of dissimilar elements: In Mendeleev’s periodic table, chemically similar elements like Cu and Hg, Au and Pt, Ag and Tl, Ba and Pb, etc have been placed in separate groups while other dissimilar elements have been placed in same group. For example, coinage metals like Cu, Ag, and Au are lesser reactive metals are placed together with higher reactive metals like Li, Na, K.
6. Cause of periodicity: It does not explain the cause of periodicity. Because of this, atomic weight is not a good basis for the classification of elements.

Modern Periodic Table

• Modern periodic table is based on atomic number.
• Modern periodic law was proposed by Moseley.

Modern periodic law : Modern periodic law states that “the physical and chemical properties of elements are periodic functions of their atomic number.”

It means that if the elements are arranged in order of increasing atomic number, the elements with similar properties are repeated after a regular interval.

Long form/ Extended form/ Present form of periodic table:

It was constructed by Bohr based on atomic number and Bohr’s- Bury electronic configuration concept.

Main features of modern periodic table:

Long form or extended form of periodic table (modern periodic table) was constructed by Bohr and is also called Bohr’s periodic table. This is an improved and extended form of Mendeleev’s periodic table. Main features of the modern periodic table are as follows:

1) There are 18 vertical columns called groups.

• The elements of group 1, 2 and 13 to 17 are called typical or representative elements.
• The elements of group 3 to 12 are known as transition elements.
• Elements of group 18 are called noble gases

2) There are 7 horizontal rows called periods. They are denoted by 1, 2, 3, 4, 5, 6, and 7.

3) 14 elements after Lanthanum(La) i.e. elements with atomic number 58 to 71 (Ce to Lu) are called lanthanides whereas, the 14 elements after Actinium(Ac) i.e. elements with atomic number 90 to 103 (Th to Lr) are called actinides. These elements are collectively called f-block elements or inner transition elements. These elements are given two separate rows below the main periodic table.

4) Elements are classified into s-block, p-block, d-block and f-block.

5) Metals and non-metals are separated from each other.

Merits (Advantages) of modern periodic table:

The modern periodic table has overcome the drawbacks of Mendeleev’s periodic table by choosing atomic number as the basis of classification. The main advantages of modern periodic table are as follows:

1. Atomic number basis: Elements are arranged on the basis of atomic number (number of protons), a fundamental property, rather than atomic mass which can be less consistent for isotopes of elements.
2. Position of isotopes: Atomic number of isotopes is same, so different isotopes can be placed at same place in periodic table. Thus position of isotopes is completely justified.
3. Proper solution of Mendeleev’s misfit points: The position of Ar, Co and Te before K, Ni and I respectively in Mendeleev’s table was not according to his periodic law. Long form of periodic table justified this anomaly by choosing atomic number as the basis of classification.
4. Separate position for subgroups: Separate positions for subgroups A and B is provided in modern periodic table. Separation of subgroups removed the anomalies found in Mendeleev’s periodic table such as grouping of chemically dissimilar elements and separation of chemically similar elements.
5. Separation of metals and non-metals: Metals are placed on the left and non-metals are placed on the right side of the periodic table.
6. Division of elements into four blocks: Division of elements into s, p, d, and f-blocks based on their electronic configuration has made their study easier.
7. Successful to explain periodicity: Modern periodic table has been successful to explain the periodicity in certain atomic properties like atomic radius, ionization potential, etc.

Demerits (Defects) of long form of periodic table:

1. Position of hydrogen is still controversial.
2. Position of helium along with the p-block elements is not completely justified as its electronic configuration is 1s².
3. Lanthanides and actinides are still not placed in main body.
4. Isotopes have different physical properties but have same place in periodic table.

Classification of elements in periodic table

Bohr’s Classification: Depending on the number of incomplete shell in an atom, the elements in the modern periodic table can be classified into four types.

1. Inert gas elements:

• These elements have completely filled ultimate(valence) shell .
• General electronic configuration is ns²np⁶
• Because of most stable configuration, they are very less reactive. Hence, known as noble gas or inert gas elements.
• These elements are present in ‘0’ group or 18^th group.

2. Representative/main group/normal elements:

• These elements have incomplete valence (ultimate) shell.
• These elements lie in group IA to VIIA
• Elements of group IA and IIA are known as alkali metals and alkaline earth metals respectively.
• Elements of group VA, VIA and VIIA are known as pnicogen, chalcogen and halogen family.

3. Transition elements:

• These elements have incompletely filled ultimate (n) and penultimate (n-1) shell.
• These elements lie in group IB to VIIB and group VIII.
• The name transition is due to their properties lies between highly reactive metals on the left side and non-metals on the right side.
• General outer electronic configuration is : (n-1)d^1-10ns^1-2

4. Inner transition elements:

• These elements have incompletely filled ultimate (n), penultimate (n-1) and antepenultimate (n-2) shell.
• These elements lie in group IIB and period 6^th and 7^th.
• These are 28 in number.
• General outer electronic configuration is : (n-2)f^1-14(n-1)d^0-1ns²
• Electronic configuration of Lanthanides is 4f^1-14,5d^0-1,6s² and Actinides is 5f^1-14,6d^0-1,7s²

Classification of elements into blocks:

On the basis of sub-shell (orbital) in which last (differentiating) electron enters, the elements are classified into four blocks: s, p, d and f-blocks.

1. s-block elements:

• Elements in which the last (differentiating) electron enters into the s-orbital of the outermost shell are called s-block elements.
• S-block consists of elements of group IA (alkali metals) and IIA (alkaline earth metals).
• These elements are very reactive metals.
• They are very soft and malleable metals.
• They have low ionization energy and are highly electropositive.
• They have low melting and boiling point.
• General outer electronic configuration is ns^1-2
• Alkali metals have largest atomic size in corresponding periods.
• Their hydroxides are strong bases.
• Most of the s-block elements (except Be and Mg) impart characteristics colour to the flame (i.e. flame test).

2. p-block elements:

• Elements in which the last (differentiating) electron enters into the p-orbital of the outermost shell are called p-block elements.
• General outer electronic configuration is ns²np^1-6
• p-block consists of elements of group IIIA (13), IVA(14), VA(15), VIA(16), VIIA(17) and zero(18).

IIIA (13) = Boron family

IVA (14) = Carbon family

VA (15) = Nitrogen family (pnicogen family)

VIA (16) = Oxygen family ( Chalcogen family >> ore forming family)

VIIA (17) = Halogen family (salt forming family)

Zero (18) = inert gases/ noble gases/ rare gases/ aerogens

3. d-block elements:

• Elements in which the last (differentiating) electron enters into the d-orbital of the penultimate shell are called d-block elements.
• d-block consists of the elements of groups IIIB(3), IVB(4), VB(5), VIB(6), VIIB(7), VIII(8-10), IB(11) and IIB(12).
• These are hard metals with high melting and boiling points.
• They are called transition elements as they exhibit transition behavior intermediate between the properties of s-and p-block elements.
• General outer electronic configuration is : (n-1)d^1-10ns^1-2
• They show variable oxidation state.
• Most of them form coloured salts and their ions or compounds are paramagnetic in nature. These properties are due to presence of unpaired electrons. For example, CuSO₄.5H₂O is blue in colour.
• Most transition elements and their compounds possess catalytic properties. Eg. Fe, Ni, Mo, Pt, etc.

4. f-block elements: Inner transition elements

• Elements in which the last (differentiating) electron enters into the f-orbital of the ante-penultimate shell are called f-block elements.
• These elements lie in group IIB and period 6^th and 7^th.
• These are 28 in number.
• General outer electronic configuration is : (n-2)f^1-14(n-1)d^0-1ns²
• Electronic configuration of Lanthanides is 4f^1-14,5d^0-1,6s² and Actinides is 5f^1-14,6d^0-1,7s²
• They have high melting and boiling point.
• They are heavy metals.
• They show variable oxidation state, commonly +3 state.
• They form coloured salts.
• Lanthanides (4f-series) are called rare earth elements since they occur rarely in the earth crust.
• Actinides (5f-series) are radioactive elements.

Nuclear charge, Shielding effect and Effective nuclear charge

Nuclear charge:

• The total positive charge present in the nucleus is called the nuclear charge.
• Its value is always positive and depends on the number of protons present in the nucleus.
• It is denoted by the letter ‘Z’.
• For example, the value of Z for oxygen is +8.

Shielding effect and Effective nuclear charge:

• In multi-electron atoms, the electrons in the valence shell are pulled by the nucleus and repelled by the electrons of inner shells. Thus, outermost electrons experience less attraction from the nucleus under the combined effect of attractive and repulsive force acting on the valence electrons. This effect is called shielding effect or screening effect.
• Thus larger the number of electrons in inner shell, the larger will be the screening effect.
• The actual charge felt by the valence electron as a result of shielding effect is called an effective nuclear charge (Z_eff).

• Effective nuclear charge (Z_eff) = Total nuclear charge (Z) – Screening constant (s)
• Value of effective nuclear charge is always less than that of the nuclear charge.
• For hydrogen, the effective nuclear charge is equal to the nuclear charge as there is no screening effect.

Periodic trends and periodicity (Atomic properties)

Periodicity of elements:

• When the elements are arranged in the modern periodic table in order of increasing atomic number, the occurrence of similar properties of elements after a definite interval is termed as periodicity of an element.
• These properties include atomic radius, ionization potential, electron affinity, electronegativity, etc.

Causes of periodicity:

• The cause of periodicity in properties is due to the same outermost shell electronic configuration coming at regular intervals.
• In the periodic table, elements with similar properties occur at intervals of 2, 8, 8, 18, 18 and 32. These numbers are called magic numbers.

The variation of some properties along with group and period are described below:

Atomic radii (size)

• Atomic radius is the distance from centre of the nucleus to the outermost shell of the electrons.
• Atomic radius cannot be measured directly because the atom cannot be isolated to determine its radius.

It can be measured indirectly from bond length measurement.

• The atomic radii are expressed in terms of covalent radii, metallic radii, Vander Waal’s radii and ionic radii.
• For most of the elements, atomic radii is measured in terms of covalent radii whereas, Vander Waal’s radii is measured for noble gases.

1. Covalent radii: The half of the distance between two nuclei in a homonuclear diatomic molecule attached to single covalent bond is called covalent radii.

For example, the bond length of H2 molecules is 0.74 Å. According to definition, covalent radius is 0.74/2 = 0.37 Å.

2. Metallic radii: The half of the distance between two nuclei of atoms attached by metallic bond in metals is called metallic radii.

3. Vander Waal’s radii: The half of the distance between two non-bonded nuclei of atoms attached by Vander Waal’s force of attraction is called Vander Waal’s radii.

The strength of various bonds is:

Covalent bond > Metallic bond >> Vander Waal’s force of attraction.

Therefore, bond length increases in the order:

Covalent radii < Metallic radii << Vander Waal’s radii

Variation of atomic radii in the periodic table:

In a group:

• In the group, from top to bottom the nuclear charge increases as well as there is increase in principle quantum number or number of shell (orbit).
• Effect of adding the new shell is larger than that of increase in nuclear charge.
• The effective nuclear charge per electron decreases.
• Hence, atomic radius (size) increases.
• Example : Atomic radii of alkali metals is: Li < Na < K < Rb < Cs

In period:

• In the period, from left to right the nuclear charge increases and electrons are added to the same shell.
• The effective nuclear charge per electron increases.
• Hence, atomic radius (size) decreases.
• Same period inert gas has highest atomic radii due to presence of Vander Waal’s radius.

Ionic radii: The ionic radius is the radii of the ions in crystal.

1. Cationic radius:

• The cation is formed by removal of one or more electron from an atom.
• The effective nuclear charge per electron in cation is more than the parent atom.
• Hence, the size of cation is smaller than that of parent atom.

Example: Fe > Fe⁺² > Fe⁺³

2. Anionic radius:

• The anion is formed by addition of one or more electron to an atom.
• The effective nuclear charge per electron in anion is less than the parent atom.
• Hence, the size of anion is larger than that of parent atom.

Example: O^-2 > O^– > O

Size of isoelectronic species:

• Those species having same number of electrons but different nuclear charge are called isoelectronic species.
• In isoelectronic species, size (radii) decreases with the increase in nuclear charge.

Example: N^-3 > O^-2 > F^– > Ne > Na⁺ > Mg⁺² > Al⁺³

Ionization energy (I.E.) / Ionization potential

The minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state to produce a cation is called ionization energy.

M (g) + I.E.  → M⁺ (g) + e^–

Note: It is an endothermic process (∆H = +ve) and measured in electron volt (eV) or KJ/mole

Successive ionization energies:

The term ionization enegy (I.E.) is in place of first ionization energy. The energy required to remove second, third, and fourth electrons are called second, third and fourth ionization energies respectively and are denoted by IE₂, IE₃ and IE₄.

M (g) + IE₁ → M⁺ (g) + e^–

M⁺ (g) + IE₂ → M²⁺ (g) + e^–

M²⁺ (g) + IE₃ → M³⁺ (g) + e^–

Factors affecting I.E.

1. Atomic size: Ionization energy decreases with the increase in atomic size.

2. Nuclear charge: Ionization energy increases with the increase in nuclear charge.

3. Shielding or Screening effect: If the shielding or screening effect of the inner electrons increases then ionization energy decreases.

• In multi-electron atoms, the electrons in the valence shell are pulled by the nucleus and repelled by the electrons of inner shells. Thus, outermost electrons experience less attraction from the nucleus under the combined effect of attractive and repulsive force acting on the valence electrons. This effect is called shielding effect or screening effect.
• The actual charge felt by the valence electron as a result of shielding effect is called an effective nuclear charge (Z_eff).
• With the increase in shielding effect, effective nuclear charge (Z_eff) decreases and hence I.E. decreases.

4. Penetration power of sub shell:

• More penetrating (i.e. more closer) are the sub-shells of a shell to the nucleus more tightly the electrons are held by the nucleus and more is the I.E.
• The penetrating power follows the order : s > p > d > f

5. Electronic configuration:

Half filled and completely filled orbitals are more stable than others and hence more energy is needed to remove an electron from such atoms. Thus, more stable the electronic configuration, the greater will be the I.E.

• Inert gases have highest I.E. due to completely filled orbital. ‘He’ has highest I.E. in the periodic table.
• Elements like Be (1s²,2s²) and Mg (1s²,2s²,2p⁶,3s²) have slightly higher I.E. due to extra stability of fully filled s-orbitals.
• Elements like N (1s², 2s², 2p³) and P (1s²,2s²,2p⁶,3s²,3p³) have higher I.E. due to extra stability of half-filled p-orbitals.

Variation of Ionization energy in the periodic table:

In group: From top to bottom in a group, atomic size increases and shielding effect also increases . Hence, ionization energy gradually decreases.

For example: Ionization energies of alkali metals is : Li > Na > K > Rb > Cs

In period: From left to right in a period, nuclear charge increases and atomic size decreases, so, there is gradual increase in I.E.

However, some elements show irregularities in the general trend. This may due to the extra stability of half-filled and completely filled electronic configurations.

For example: the variation of I.E. among the elements of II^nd period is:

Li < Be > B < C < N > O < F < Ne

Electron Affinity

The amount of energy released when an electron is added to an isolated gaseous atom in its ground state to form a gaseous anion is called electron affinity (EA).

X (g) + e^–  → X^– (g) + energy (EA)

Note: It is measured in electron volt (eV) or KJ/mole.

Factors affecting electron affinity:

1. Atomic size: Electron affinity decreases with the increase in atomic size.
2. Nuclear charge: Electron affinity increases with the increase in nuclear charge.
3. Screening effect: Electron affinity decreases with the increase in screening effect.
4. Electronic configuration: Elements having stable electronic configuration like half filled and completely filled orbitals have EA either very low or almost zero as they do not accept additional electrons so easily.

• EA of inert gases is zero due to completely filled orbitals.
• EA of alkaline earth metals is almost zero due to completely filled s-orbital.
• EA of N, P is very low due to half filled orbital.

Variation of electron affinity in the periodic table:

In period: From left to right in a period, nuclear charge increases and atomic size decreases, so, there is gradual increase in EA.

However, some elements show irregularities in the general trend. This may due to the extra stability of half-filled and completely filled electronic configurations.

For example: the variation of EA among the elements of II^nd and III^rd period is:

In 2^nd period : Ne < Be < N < Li < B < C < O < F

• Halogens possess maximum electron affinity due to small size and maximum effective nuclear charge and after gaining one electron, they attain stable inert gas configuration.

In group: From top to bottom in a group, atomic size increases and shielding effect also increases . Hence, EA gradually decreases.

For example: EA of alkali metals is : Li > Na > K > Rb > Cs

• However, the electron affinity of fluorine is lower than chlorine. Due to small size of fluorine, the incoming electron feels more repulsion and less attraction. In case of chlorine, incoming electron feels less repulsion and more attraction than in fluorine. Hence, the EA of fluorine is lower than that of chlorine.

The EA order of halogens is : Cl > F > Br > I

This type of anomaly is also observed in chalcogens (group 16) i.e. S>O>Se>Te

Anomalous Electron affinity:

• Electron affinity is …………….
• Generally, From left to right in a period, nuclear charge increases and atomic size decreases, so, there is gradual increase in EA. From top to bottom in a group, atomic size increases and shielding effect also increases . Hence, EA gradually decreases.
• However, some exceptions in the general trend in EA are found, which is called anomalous EA. Some of the anomalies are:

1. Zero and very low EA : Elements having stable electronic configuration like half filled and completely filled orbitals have EA either very low or almost zero as they do not accept additional electrons so easily.

• EA of inert gases is zero due to completely filled orbitals.
• EA of alkaline earth metals is almost zero due to completely filled s-orbital.
• EA of N, P is very low due to half filled orbital.

2. Halogens have highest EA: Halogens possess maximum electron affinity due to small size and maximum effective nuclear charge and after gaining one electron, they attain stable inert gas configuration.

3. Electron affinity of fluorine is lower than chlorine: Due to small size of fluorine, the incoming electron feels more repulsion and less attraction. In case of chlorine, incoming electron feels less repulsion and more attraction than in fluorine. Hence, the EA of fluorine is lower than that of chlorine.

The EA order of halogens is : Cl > F > Br > I

This type of anomaly is also observed in chalcogens (group 16) i.e. S>O>Se>Te

Eletronegativity (E.N.)

• Electronegativity of an element is defined as the relative tendency of an atom in a molecule to attract shared pair of electrons towards itself.
• It has no unit.
• Higher the difference in electronegativity, more the polarity in the bond.

Factors affecting Electronegativity:

1. Atomic size: Smaller the size of an atom, the greater is its tendency to attract the shared pair of electrons towards itself. Hence,the electronegativity increases with a decrease in size of the atom.

2. Effective nuclear charge (Z_eff): EN increases with the increase in Z_eff.

3. Ionization energy and Electron affinity: Higher the value of I.E. and EA, higher is the EN.

4. Number and nature of atoms bonded to it: EN of an element depends upon the number and nature of the atoms to which it is bonded. For example, the EN of phosphorous in PCl₅ is higher than in PF₅, since fluorine is more electronegative than chlorine.

5. Type of hybridization: The EN increases as the s-character in hybrid orbital increases.

For example: EN of carbon in methane, ethane and ethyne is:

Ethyne > Ethene > Methane

6. Charge on the ion: Cation is smaller in size while anion has larger size as compared to that of parent atom. Hence, EN increases with the increase in +ve charge and decrease in negative charge.

For example:

Fe < Fe⁺² < Fe⁺³

O^-2 < O^– < O

Variation of electronegativity in the periodic table:

In group: From top to bottom in a group, atomic size increases and shielding effect also increases. Hence, EN gradually decreases.

For example: EN of alkali metals is : Li > Na > K > Rb > Cs

EN of halogens is : F > Cl > Br > I >At

In period: From left to right in a period, nuclear charge increases and atomic size decreases, so, there is gradual increase in EN.

For example: EN order of second period : Li < Be < B < C < N < O < F

Metallic character [Electropositive character]

• The tendency of an element to lose an electron to form a cation is called electropositive character.
• The electropositive character of metal is called a metallic character.
• Lesser the value of ionization energy, more will be the metallic character and vice-versa.

Variation of metallic character in the periodic table:

In group: The metallic character of elements increases in going from top to bottom in a group. This is due to increase in size of the atom.

For example: Group 1: Li < Na < K < Rb < Cs

In period: The metallic character of elements decreases in going from left to right in a period. This is due to increase in effective nuclear charge.

For example: Period 3: Na > Mg > Al

Diagonal relationship

Similarities between certain pairs of elements that are diagonally adjacent to each other in the second and third period of the periodic table is called diagonal relationship.

Diagonal relationship is due to,

1. Almost same electronegativity (main cause)
2. Almost same atomic size.

Note: Bridge elements– Bridge elements are the elements of the second period of the periodic table. These elements show a relationship with the third-period,  which are diagonal.

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• Chemical reactions are the processes by which chemicals interact to form new chemicals with different compositions.
• Simply, a chemical reaction is the process where reactants are transformed into products.

• A reactant is a substance that is present at the start of a chemical reaction and a product is a substance that is present at the end of a chemical reaction.

Types of chemical reactions

Following are the principal types of chemical reactions :

Combination or synthesis reaction

In combination reaction, two or more substances (i.e. reactants) unite together to produce a new substance (i.e. product).

Examples :

Decomposition or analysis reaction

In this reaction, a single compound is decomposed or broken into two or more products.

Examples :

Displacement or Replacement or Substitution reaction

The process of displacing one of the constituents of a compound and occupying its position by another constituent is called displacement reaction or replacement reaction.

It is of two types – single displacement and double displacement reactions.

Single displacement reaction :

In this reaction, one element displaces another element from a compound.

Examples :

Double displacement reaction :

In this reaction, there is mutual exchange of ions between two pairs of reacting compounds.

Examples :

Substitution reaction usually occurs in solution and reaction is also known as precipitation reaction.

The reaction in which formation of an insoluble solid (precipitate) takes place by reacting two or more than two aqueous reagents is called precipitation reaction.

Acid – base reaction or Neutralization reaction

• A chemical reaction in which acid reacts with base to give salt and water is called acid base reaction or neutralization reaction.
• It is also a double displacement reaction. Examples :

Hydrolysis reaction

• A chemical reaction which is done by the action of water on any substance to form product is known as hydrolysis.
• It is the interaction of ions of salt with water to produce acidic or basic solution. Examples:

a) Sodium carbonate dissolves in water to produce strong base and weak acid (i.e. basic solution).

b) Ferric chloride is dissolved in water to produce strong acid and weak base (i.e. acidic solution).

Rearrangement or isomerisation reaction

• Formation of a new compound due to the internal change in position of atoms or groups in the molecule without any loss or gain of atoms is called rearrangement reaction.
• In this reaction, the molecular formula of the reactants and products remains same but the structure will be different.

Examples :

Polymerization reaction

The reaction in which the small molecules (monomers) combine to form giant molecule (polymer) is called polymerization reaction. It is of two types.

a. Addition polymerization reaction

b. Condensation polymerization reaction

a. Addition polymerization reaction :

The reaction in which polymer is formed by combination of monomers without elimination of smaller molecules is called addition polymerization reaction.

b. Condensation polymerization :

The reaction in which polymer is formed by combination of monomers with elimination of smaller molecules like water, CO₂, etc. is called condensation polymerization.

Example: pyrosulphuric acid (oleum) is obtained by combination of sulphuric acid molecules with the removal of water molecule.

Redox reaction

• A chemical reaction involving both oxidation and reduction process is called redox reaction.
• Redox reactions are the chemical reactions in which the reactants undergo a change in their oxidation states. Examples :

The loss of electrons and then increase in the oxidation state (number) of a reactant is called oxidation.

The gain of electrons and then decrease in the oxidation number of a reactant is called reduction.

Reversible and Irreversible reactions

• Chemical reactions in which the products formed interact (react) with each other under suitable conditions to give back the original reactants are called reversible reactions.
• It is represented by two half arrows between the reactants and the products i.e. one arrow pointing towards products and other pointing towards the reactants. Examples :

• Chemical reactions in which the products formed do not react with each other to form the original reactants are called irreversible reactions. Examples :

Exothermic and Endothermic reactions

• A chemical reaction which takes place with the evolution of heat is called an exothermic reaction. Examples :

• A chemical reaction which takes place with the absorption of heat is called an endothermic reaction. Examples :

References

• https://www.thoughtco.com/endothermic-and-exothermic-reactions-602105
• https://chem.libretexts.org/Courses/Valley_City_State_University/Chem_121/Chapter_5%3A_Introduction_to_Redox_Chemistry/5.3%3A_Types_of_Chemical_Reactions#:~:text=The%20five%20basic%20types%20of,into%20more%20than%20one%20category.
• https://www.brighthubengineering.com/thermodynamics/4616-what-are-reversible-and-irreversible-processes/
• https://www.britannica.com/science/substitution-reaction

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Organic compounds are defined as the hydrocarbons (compounds containing carbon and hydrogen) and their derivatives in which covalently bonded carbon is an essential constituent.

Organic compounds are classified as :

1. Open chain organic compounds :

Organic compounds in which the terminal C-atoms are not joined together are called open chain compounds. Eg.

The open chain organic compounds can be further classified as,

Alkanes : Alkanes are the saturated hydrocarbons with general formula C_nH_2n+2. They contain only carbon-carbon and carbon-hydrogen single bonds in their molecules. For example:

• Alkanes are also called paraffins (because they have a little affinity towards a general reagent. In other words, alkanes are less reactive substances. They undergo reactions under drastic conditions)

Note : Alkanes are called saturated because all the possible sites (i.e. 4) are bonded with other atoms.

But in alkenes and alkynes there is a possibility of addition of atoms or groups so they are called unsaturated.

Alkenes : Alkenes are unsaturated hydrocarbons with general formula C_nH_2n. They contain at least one carbon to carbon double bond in their molecules. For example:

• Alkenes are also called olefins ( i.e. oil forming because they form oily liquids on reaction with chlorine gas.

Alkynes : Alkynes are unsaturated hydrocarbons with general formula C_nH_2n-2. They contain at least one carbon to carbon triple bond in their molecules. For example:

2. Closed chain (cyclic) organic compounds :

Organic compounds in which the terminal carbons are joined together to form a cyclic structure are called closed chain or cyclic organic compounds. For exampne:

Cyclic organic compounds are further classified as – homocyclic and heterocyclic organic compounds.

Homocyclic compounds : Cyclic organic compounds in which the ring forming atoms are only carbon are called homocyclic or more specifically carbocyclic compounds.

Homocyclic compounds can be further classified as – Alicyclic and Aromatic compounds.

Alicyclic compounds :

Closed chain organic compounds whose properties are similar to open chain aliphatic compounds are called alicyclic compounds. For example:

Aromatic compounds :

Benzene and those cyclic compounds that chemically behave as benzene are called aromatic compounds. For example:

Aromatic compounds obey Huckel’s rule.

Note : Huckel’s rule :

Huckel’s rule states that a cyclic and planar molecule is aromatic if it contains 4n+2 delocalized π electrons, where n = 0,1,2,3,4,etc.

Examples : Benzene

Benzene is cyclic and planar and has cyclic overlap of p-orbitals. There are 3 double bonds i.e. 6 delocalized π – electrons, which is consistant with Huckel’s rule.

i.e. 4n+2 = 6

4n= 4

n = 1(which is an integer)

Therefore, benzene is an aromatic compound. It will show aromaticity.

Heterocyclic compounds :

Cyclic organic compounds in which at least one heteroatom (i.e. atom other than carbon eg. N, O or S ) is present as one of the ring forming atoms are called heterocyclic compounds. Examples :

Formula of organic compounds

• Molecular formula :

It represents actual number of atoms of all the elements present in one molecule of the compound. For example:

methane = CH₄

ethane = C₂H₆

ethene = C₂H₄

benzene = C₆H₆ , etc.

• Empirical formula :

It represents simple whole number ratio of atoms of all the elements in one molecule of the compound. For example:

ethane = CH₃

ethene = CH₂

benzene = CH , etc.

• Electron-dot formula : In this formula valence electrons are represented by dots placed around the chemical symbol. It is also called Lewis formula. For example:

• Structural formula : It indicates how the atoms are bonded in a molecule of the compound. For example:

• Contracted or condensed formula :

It is the structural formula in contracted form to save space and time.

• Bond – line formula :

In this type of formula, carbon and hydrogen atoms are not shown and only hetero atoms are shown. The point of intersection represents carbon along with required number of hydrogen to satisfy the valency of carbon. For example:

• Spatial formula :

This formula represents the three dimensional shape or arrangement of atoms in the molecule. For example :

Functional group

An atom or group of atoms in a molecule which largely determines the chemical properties of the organic compounds is known as functional group. All the compounds having same functional group show similar properties and constitute a class or a family.

For example: organic compounds having – OH as functional group constitutes a class of compounds called alcohol.

Some other examples of functional group are :

Homologous series

The series of organic compounds having same general formula and similar chemical properties but different physical properties in which one member differs from other member by single – CH₂ unit is known as homologous series. For example:

Each member of homologous series is called homologue and phenomenon of making homologous series is called homology.

Characteristics of homologous series :

• All the members of homologous series have same functional group.
• All the members of homologpous series have same chemical properties.
• All the members of homologous series can be prepared by a common method of preparation. Eg.

• All the members of homologous series can be represented by same general formula. Eg.

C_nH_2n+2 = Alkane

C_nH_2n = Alkene

C_nH_2n-2 = Alkyne

C_nH_2n+1OH = Alcohol, etc.

• Their molecular masses increases gradually hence their physical properties (eg. melting point and boiling point) changes gradually.
• Each member differs from the adjacent member by methylene (-CH₂-) unit.

Q) Write down the 1^st, II^nd, III^rd and IV^th homologue of aldehyde homologous series.

I^st homologue = HCHO

II^nd homologue = CH₃CHO

III^rd homologue = CH₃CH₂CHO

IV^th homologue = CH₃CH₂CH₂CHO

See the IUPAC Nomenclature of Organic compounds ……

Also see the Isomerism …

Objective questions

1. Cyclic organic compounds possessing the properties of aliphatic compounds are called ___ compounds.

a. Aromatic        c. Carbocyclic

b. Homocyclic    d. Alicyclic

2. The formula which represents the simple whole number ratio of different atoms present in one molecule of a compound is known as :

a. Molecular formula   c. Condensed formula

b. Empirical formula    d. Electron dot formula

3. An organic compound has empirical formula CH₂O and molecular weight 90. It’s molecular formula will be :

a. C₆H₁₂O₆    c. C₂H₄O₂

b. C₃H₆O₃      d. C₃H₉O₆

4. General formula of an alkene is :

a. C_nH_2n       c. C_nH_2n-2

b. C_nH_2n+2    d. C_nH_2n-1

5. A hydrocarbon is found to contain 81.80% carbon and 18.20% hydrogen. It’s empirical formula will be :

a. C₄H₈    c. C₃H₈

b. C₂H₆    d. C₃H₆

6. If two compounds have same empirical formula but different molecular formula, they must have :

a. Different percentage composition

b. Different molecular weight

c. Same viscosity

d. Same vapour density

7. Which of the following is an aromatic compound :

a. Benzene hexachloride   c. Cyclobutane

b. Cyclohexane     d. Toluene

8. Which of the following is a heterocyclic aromatic compound :

a. Nephthalene     c. Furan

b. Benzene hexachloride   d. Toluene

9. Which of the following belongs to a homologous series ?

a. Methanol, Ethanol, ethanoic acid

b. Propane, Propene, propyne

c. Butane, 2-methylpropane, 2-methylbutane

d. Chloroathane, 2-chloropropane, 1-chlorobutane

10. Which of the following is not true about homologous series ?

a. Adjacent members of group differ by one –CH2 group.

b. Adjacent members of group differs by a mass of 14 amu.

c. Members of a group have same chemical and physical properties.

d. Members of a group can be prepared by same general methods.

a. Isomers    c. allotropes

b. Homologues   d. None

12. Alkanes are also called :

a. Paraffins    c. Acetylene

b. Olefins     d. Both ‘a’ and ‘b’

Answer :

1 – d   2 – b   3 – b

4 – a   5 – c   6 – b

7 – d   8 – c   9 – d

10 – c   11 – b   12 – a

References

• Sthapit, M.K., Pradhananga, R.R., Foundations of Chemistry, Vol 1 and 2, Fourth edition, Taleju Prakashan, 2005.
• Bahl, B.S., A., Advanced Organic Chemistry, S. Chand and company Ltd, New Delhi, 1992.
• Finar, I. L., Organic Chemistry, Vol. I and Vol. II, Prentice Hall, London, 1995.
• Ghosh, S.K., Advanced General Organic Chemistry, Second Edition, New Central Book Agency Pvt. Ltd., Kolkatta, 2007.
• https://socratic.org/organic-chemistry-1/ways-to-draw-and-represent-molecules/condensed-structure
• https://www.britannica.com/science/homologous-series

The post Organic compounds Classification, Functional group and Homologous series appeared first on Online Chemistry notes.

Ozone is a tri-atomic molecule of oxygen having molecular formula O₃ and molecular weight 48.

There is a layer of ozone in the atmosphere from 12 to 24 km from the surface of the earth. Ozone is formed naturally in the stratosphere by the action of ultraviolet radiation from the sun on oxygen.

Ozone layer absorbs ultraviolet radiations from the sun and protects the living beings on the earth from the harmful effect of ultraviolet radiation.

Preparation of ozone from oxygen

Ozone is prepared by passing silent electric discharge through pure and dry oxygen in an apparatus called ozonizer. The produced gas is called ozonized oxygen that contains 10-15% ozone.

Structure of ozone

Experimental evidences (like electron diffraction, microwave studies) indicate that ozone molecule is angular with bond angle 116⁰4’’. The bond lengths are equal (1.279 Å ). This indicates ozone molecule is resonance hybrid of structure (I) and (II).

Ozone layer depletion (ozone hole)

Ozone hole means a severe depletion of ozone in the region of ozone layer through which UV rays from the sun can penetrate to the earth causing devastating effects.

Causes of ozone layer depletion :

The main causes responsible for ozone layer depletion are as follows:

Chlorofluorocarbons :

The main cause of ozone layer depletion is manmade chlorofluorocarbons (CFCs). CFCs also known as Freons are compounds containing chlorine, fluorine and carbons. They are extensively used by human beings as coolants in refrigerator, air conditioner etc.

The common CFC’s are CFCl₃ (Freon 11), CCl₂F₂ (Freon 12), CHClF₂ (Freon 22) etc.

Because of high stability, freons remain in the environment for a long time and reaches to the stratosphere. In stratosphere, CFCs absorbs UV light and decomposes to give chlorine atom which destroys the ozone molecules by chain reaction as follows:

It is estimated that one molecule of CFC can destroy 100,000 molecules of ozone. Therefore, the use of CFC’S is banned nowadays.

Unregulated rocket launches and nuclear tests :

Researchers say that the unregulated launching of rockets and nuclear tests result in much more depletion of ozone layer than the CFCs do. If not controlled, this might result in a huge loss of the ozone layer.

Nitrogenous Compounds :

The nitrogenous compounds such as NO₂, NO, N₂O are highly responsible for the depletion of the ozone layer.

Cleaning products and fire extinguishers :

Cleaning products and fire extinguishers contain or produce ozone depleting substances like halons (eg. Bromotrifluoromethane – CBrF₃), carbon tetrachloride, hydrofluorocarbons (organic compounds containing hydrogen and fluorine eg. CHF₃), etc.

Natural Causes :

The ozone layer has been found to be depleted by certain natural processes such as stratospheric winds. But it does not cause more than 1-2% of the ozone layer depletion.

The volcanic eruptions are also responsible for the depletion of the ozone layer.

Effects of ozone layer depletion:

• Direct exposure to ultraviolet radiations causes health problems such as skin and eye cancer, weekend immune system, etc. in human and other animals.
• Strong ultraviolet rays may lead to minimal growth, flowering and photosynthesis in plants.
• Synthetic polymers, naturally occurring biopolymers, as well as some other materials of commercial interest are adversely affected by UV radiation.

Control measures of ozone layer depletion:

Some points that would help to prevent ozone layer depletion are as follows:

• Avoid the use of dangerous gases like CFCs (chlorofluorocarbons), halogenated hydrocarbons, nitrogen oxides, etc.
• Minimize the use of vehicles – The vehicles emit a large amount of green house gases that lead to the ozone layer depletion. Therefore, the use of vehicles should be minimized as much as possible. Promote the use of electric vehicles and bicycles should be promoted.
• Do not use cleaning products containing halogens (eg.CHF₃). We can replace these dangerous substances with safe compounds like bicarbonates.
• Maintain air conditioners, as their malfunctions cause chlorofluorocarbons to escape into the atmosphere.
• Rocket lunching and nuclear tests should be regulated .

Test of ozone

These two reactions can help to identify ozone:

Action with mercury (Mercury tailing) :

When mercury comes in contact with ozone, ozone oxidizes mercury to mercurous oxide. The meniscus of mercury is lowered and it leaves a tail when allowed to fall through an inclined surface. This property of mercury is called tailing of mercury.

Action with KI and starch :

When ozone is passed into the solution of KI and starch, ozone oxidizes KI to iodine and iodine gives blue colour with starch.

These reactions can be used for the test of ozone.

Uses of ozone

• It is used as germicide and disinfectant for the sterilization of water due to its oxidizing nature.

• It is used to prepare organic compounds like aldehydes and ketones.
• It is used for bleaching delicate articles like silk, starch, wax, etc.

• It is used for air purification at the crowded places like cinema halls and tunnel railways.

Objective Questions

1. O₂ and O₃ are:

a. Isotopes    c. Allotropes

b. Isobars     d. Isomers

2. Ozone is tested by:

a. Ag     c. Hg

b. Au     d. Zn

3. Ozone is in high concentration in :

a. Troposphere     c. Mesosphere

b. Stratosphere     d. Thermosphere

4. Tailing of mercury is caused by :

a. Cl₂      c. O₂

b. H₂      d. O₃

5. One molecule of chlorofluorocarbons can destroy upto ____ molecules of ozone.

a. 100     c. 10000

b. 1000   d. 100000

6. Montreal protocol is related to the:

a. Global warming

b. Ozone layer depletion

c. Sustainable development

d. Food security

7. Which of the following is known as freon?

a. CCl₂F₂     c. CF₄

b. CHCl₃     d. CHF₂

Answers :

1.c   2. c   3. b    4. d

5. d    6. b    7. a

References

• Agrawal, S. K., Lal, K., Advanced Inorganic Chemistry, Fifth Revised Edition, Pragati Prakashan, Meerut, 2001.
• Cotton, F. A., and Wilkinson, G., Advanced Inorganic Chemistry, Fifth edition, John Wily and Sons, Singapore, 1995.
• Day, C.M., Selbin, J., Theoritical inorganic Chemistry, second edition, Affiliated East-West Press Pvt. Ltd., New Delhi, 2002.
• https://www.sizes.com/units/dobson_unit.htm#:~:text=A%20unit%20expressing%20the%20concentration,It%20is%20named%20for%20G.%20M.%20B.
• https://www.epa.gov/ozone-pollution-and-your-patients-health/what-ozone#:~:text=Ozone%20(O3)%20is%20a%20highly,either%20good%20or%20bad%20ways.
• https://www.epa.gov/ground-level-ozone-pollution/ground-level-ozone-basics

The post Ozone – Preparation, Uses, Test, Ozone layer depletion appeared first on Online Chemistry notes.

The two or more forms of same element having similar chemical properties but different physical properties are called allotropes and the phenomenon is called allotropy. For example- diamond and graphite are two allotropes of carbon.

Allotropes of carbon, sulphur and phosphorus are described below.

Allotropes of carbon

Carbon exists on following allotropic forms:

.

Crystalline form :

1. Diamond :

Diamond is the purest form of carbon. In diamond 1 carbon atom is bonded with 4 other carbon atoms by covalent bond forming tetrahedral structure. The bond angle between carbon atoms in diamond is found to be 109.5⁰ and its bond length is 1.54 Å.

• It is transparent and sparks when light falls on it.
• It is the hardest substance known.
• It is purest form of carbon ( 100% carbon)
• It’s density is very high (3.5 at 15⁰C)
• It has very high refractive index ( 2.417)
• Its melting point is very high (3750⁰C).
• It is bad conductor of heat and electricity.

Note : Diamond is bad conductor because its electrons are not free to move due to the formation of sp³ hybrid covalent bonds between carbon atoms.

Uses of diamond :

• It is used in jewellery.
• It is used for cutting glass.

Note : Diamond is weighed in carat, 1 carat = 0.2 gm

2. Graphite :

In graphite one atom is attached with other three carbon atoms by covalent bond which forms hexagonal ring. Out of four valence electrons only three valence electrons are used to form three covalent bonds leaving one electron free. This free electron per atoms is responsible for making graphite a good conductor. The hexagonal rings of graphite form a layer structure. The two adjacent layers of graphite are held with each other by Vander Waal’s force of attraction. Distance between two layers is 3.4 Å. The bond length between two carbon atoms is 1.42 Å and bond angle is 120⁰.

• It is good conductor of heat and electricity.
• It’s melting point is 3500⁰C.
• It is resistant towards many chemical substances but it burns on strong heating.

Uses of graphite :

• It can mark the paper. So, it is used to prepare lead of pencil.
• It is used to prepare electrodes.
• It is used for the preparation of artificial diamond.

Note : Graphite can be converted into diamond at about 1600⁰C and 50,000 to 60,000 atm pressure. Because of high cost and poor quality, diamond is seldom (rarely) made artificially.

3. Fullerene :

It is latest discovered allotropic form of carbon (in 1985). The most common form of fullerene is C₆₀. It is named as Buckminster fullerene after the name of Rechard Buckminster. It contains 60 carbon atoms arranged in hexagonal and pentagonal ring which forms spherical bucky ball or soccer like structure. So, C₆₀ molecules are also called bucky balls.

• It is soluble in organic solvent.
• It is good conductor of electricity.

Note : Other forms of fullerene are C₃₂ , C₅₀ , C₇₀ , C₇₆ , C₈₄ , etc.

Uses of fullerene :

• It is used to make superconductor (due to its reasonable electric conductivity).
• It is used to make carbon nanotubes.
• It is used as molecular sieve.

Amorphous forms :

1. Coal : It is black or almost black solid combustible substance. It is naturally formed by the partial decomposition of wood or vegetable matter in presence of moisture at high pressure and temperature. It is used as fuel.

2. Coke : It is the residue of coal obtained after destructive distillation of coal. During this process volatile organic substances escape out. It is pure form of carbon, used as fuel as it burns without smoke. It is also used as a reducing agent in metallurgy.

3. Charcoal : It is porous form of carbon produced by the destructive distillation of organic materials like wood, sugar, bone, etc. Organic materials are heated strongly in limited supply of air. Charcoals are highly porous and used as adsorbent for absorbing toxic gases and purifying liquids.

4. Gas carbon : Gas carbon is dense form of deposited carbon on the interior part of gas retort during the manufacture of coal gas. It is good conductor of electricity and is used for making electrode in dry cell.

5. Lamp black : Lamp black is a finely divided black powdered soot obtained by burning natural gas and other carbon rich compounds in limited supply of air. It is used in manufacture of ink, black paints, boot polishes, carbon paper, etc.

Allotropes of sulphur

Sulphur exists on following allotropic forms :

Crystalline Allotropes :

1. Rhombic sulphur ( α – sulphur ) :

• It is most stable form of sulphur at ordinary temperature.
• The crystals are octahedral (eight sided).
• It has pale yellow colour.
• It is insoluble in water but soluble in carbon disulphide (CS₂).
• It’s melting point is 114⁰C.
• It is bad conductor of heat and electricity.
• Above 96⁰C, the rhombic sulphur changes to monoclinic sulphur.

2. Monoclinic sulphur ( β – sulphur) :

• It is needle shaped crystalline sulphur.
• It is stable allotrope of sulphur above 96⁰C.
• It is insoluble in water but soluble in carbon disulphide (CS₂).
• It’s melting point is 119⁰C.

Note : 96⁰C is the transition temperature at which both α and β forms can co-exist. Below 96⁰C rhombic sulphur is stable and above 96⁰C monoclinic sulphur is stable.

Amorphous allotropes :

1. Plastic sulphur ( γ – sulphur) :

• When boiling sulphur is poured into cold water, a rubber like substance is formed which is plastic sulphur.
• It is insoluble in water and CS₂.
• It is super cooled liquid and does not have sharp melting point.
• On long standing, plastic sulphur changes to rhombic sulphur.

2. Colloidal sulphur ( δ – sulphur) :

It is prepared by passing H₂S gas to the solution of oxidizing agents like SO₂, HNO₃, KMnO₄, K₂Cr₂O₇, etc.

• It is insoluble in water but soluble in CS₂.
• It has no sharp melting point.
• On long standing or heating, it changes into rhombic sulphur.

3. Milk of sulphur :

• When sulphur is boiled with milk of lime [Ca(OH)₂] then water soluble mixture of calcium pentasulphide (CaS₅) and calcium thiosulphate (CaS₂O₃) is obtained. This mixture is then treated with dil. HCl and amorphous milk of sulphur is precipitated.

• It is white solid insoluble in water but soluble in CS₂.
• It has no sharp melting point.
• It is bad conductor of heat and electricity.
• On heating it changes to rhombic sulphur.

Allotropes of phosphorus

Phosphorus exists on following allotropic forms :

Among them white and red phosphorus are more common.

White Phosphorus :

White phosphorus is manufactured by heating calcium phosphate i.e. Ca₃(PO₄)₂ with sand (SiO₂) and coke (C) .

Calcium phosphate reacts with sand to form calcium silicate and phosphorus pentoxide.

Phosphorus pentoxide in vapour form is reduced by coke and liberates phosphorus at 1500⁰C.

• It is soft, waxy white solid with garlic odour.
• Its melting point is 44⁰C and boiling point is 287⁰C.
• It is extremely poisonous.
• Its ignition temperature is 35⁰C. It burns (oxidize) in air giving yellow-green flame forming phosphorus pentoxide and trioxide. This phenomenon is called phosphorescence.

Hence, it is stored under water.

Uses of white phosphorus :

• It is to prepare poison for killing rats.
• It is used to prepare phosphine gas, phosphoric acid, etc.
• Hypophosphite prepared from white phosphorus is used in medicines as tonic.

Red Phosphorus :

• When white phosphorus is heated at about 250⁰C in an inert atmosphere( of nitrogen or carbondioxide or coal gas) for several hours then it is converted into red phosphorus.
• It is dark red powder.
• It is colourless and non-poisonous.
• Its melting point is 550⁰C and sublimes at 290⁰C in absence of air.
• It is less reactive than white phosphorus.
• It does not show the phenomenon of phosphorescence.

Uses of red phosphorus :

• It is used in the manufacture of phosphate fertilizer.
• It is used for making match stick in match industry.

Note: The tip of match stick contains combustible material Sb₂S₃ along with oxidizing agent like KClO₃, PbO₂ or K₂CrO₄ and the two sides of the match box is coated with a mixture of powdered glass (abrasive) and red phosphorus. Friction applied to the side of the match box vaporizes red phosphorus and ignites. Thus produce fire to the head of match stick.

References

• Lee, j. D., Concise Inorganic Chemistry, Fifth Edition, Joh, Wiley and Sons, Inc., 2007.
• Sarkar, R., General and Inorganic Chemistry, Second Edition, New Central Book Agency(P) Ltd., India, 2007.
• Shriver, D. F., Atkins, P. W., Inorganic Chemistry, Fifth Edition, Oxford university Press, 2010.
• Agrawal, S. K., Lal, K., Advanced Inorganic Chemistry, Fifth Revised Edition, Pragati Prakashan, Meerut, 2001.
• Cotton, F. A., and Wilkinson, G., Advanced Inorganic Chemistry, Fifth edition, John Wily and Sons, Singapore, 1995.
• https://www.simply.science/images/content/chemistry/metals_and_non_metals/oxygen_family/conceptmap/Monoclinic_sulphur.html#:~:text=Monoclinic%20sulfur%20is%20a%20crystalline,ring%20molecules%20in%20crystalline%20structure.
• https://geology.com/minerals/graphite.shtml
• https://www.sciencedirect.com/topics/engineering/red-phosphorus

The post Allotropes and Allotropy – Allotropes of Carbon, Sulphur and Phosphorus appeared first on Online Chemistry notes.

An atom is the smallest particle of an element which can take part in chemical reaction. Atom consists of three fundamental particles i.e. proton, neutron and electron. Atoms of same elements are similar in properties whereas atoms of different elements are different in properties. Example:- ‘H’ represent the atom of hydrogen.

Proton is positively charged and electron is negatively charged particle. In an atom, number of protons = number of electrons. Hence, the net charge present in an atom is zero i.e. a free atom is chargeless.

Atomic number and Mass number

Atomic number :

• Atomic number is the number of protons present in an atom.
• The modern periodic table is arranged in order of increasing atomic number.

Mass number and Atomic mass :

• Mass number is the sum of the number of protons and the number of neutrons present in an atom. It is a whole number.

Mass no. of an atom = No. of protons + No. of neutrons

• Atomic mass is the average mass of the all of the isotopes of that element. It is a decimal number.
• For example: Hydrogen has three isotopes – ₁H¹, ₁H² and ₁H³ having mass number 1, 2 and 3 respectively. Naturally occurring hydrogen contains about 99.985% of protium, 0.014% of deuterium and 0.001 % of tritium. Therefore the atomic mass of hydrogen is 1.00784 amu.
• The atomic mass of an element element is measured in atomic mass unit (amu, also known as Daltons ‘ D’or unified atomic mass unit ‘u’).
• 1amu = 1.66 x 10^-24 grams. 1gm = 6.022 x 10²³ amu ( i.e. Avogadro’s number).

Here,

• Atomic number = Number of protons = Number of electrons = 13
• Mass number = No. of protons + No. of neutrons
• No. of neutrons = Mass number – No. of protons = 27-13 = 14.

Atomic mass of first 20 elements

Isotopes

Atoms of the same element having same atomic number but different mass number (atomic mass/weight) are called isotopes. For example:

Isotopes of hydrogen :

There are three isotopes of hydrogen:

1. Protium or ordinary hydrogen
2. Deuterium or heavy hydrogen
3. Tritium or radioactive hydrogen.

Naturally occurring hydrogen contains about 99.985% of protium, 0.014% of deuterium and 0.001 % of tritium.

Isotopes have different physical properties since they differ in their mass number.

They have same chemical properties since their electronic configuration is same. However, they differ in the rate of chemical reaction. For example, D₂ reacts with Cl₂ about 13 times slower than H₂ does. The different in rate of reaction due to difference in mass of the atoms of the same element is called isotope effect.

Some other examples of isotopic elements :

Isobars

Atoms of different elements having different atomic number but same mass number are called isobars. For example :

₁₈Ar⁴⁰, ₁₉K⁴⁰ and ₂₀Ca⁴⁰

Isotones

Atoms of different elements having different atomic number and mass number but same number of neutrons are called isotones. For example :

₆C¹⁴, ₇N¹⁵ and ₈O¹⁶

Objective questions and their answers

1. Which of the following is known as heavy hydrogen?

a. Protium       c. Tritium

b. Deuterium  d. Para hydrogen

2. Which of the following is known as radioactive hydrogen?

a. Protium       c. Tritium

b. Deuterium  d. Para hydrogen

3. Least abundant isotope of hydrogen is:

a. Protium       c. Tritium

b. Deuterium  d. Heavy hydrogen

4. Diamond and graphite are :

a. Isotopes   c. Isotones

b. Isobars    d. Allotropes

5. ₆C¹⁴ and ₈O¹⁶ are :

a. Isotopes   c. Isotones

b. Isobars    d. Allotropes

6. ₆C¹⁴ and ₇N¹⁴ are :

a. Isotopes   c. Isotones

b. Isobars    d. Allotropes

7. All particles residing inside the nucleus of an atom are termed as:

a. Protons     c. Electrons

b. Neutrons  d. Nucleons

8. What makes the atomic mass fractional ?

a.Prerence of isotopes

b. Number of unpaired electrons

c. Spherical shape

d. Quantum number.

9. Which of the following are not isotopes:

a. ₁H¹ and ₁H³

b. ₁₈K⁴⁰ and ₂₀Ca⁴⁰

c. ₆C¹⁴ and ₇N¹⁴

d. Both b and c.

10. Charge present in the nucleus of an atom is :

a. Positive     c. Chargeless

b. Negative   d. Both +Ve and -Ve

11. Molecular weight of heavy water is :

a. 16    c. 20

b. 18    d. 22

Answers :

1. b   2. c   3. c

4. d [Note : different forms of same element having different properties are called allotropes]

5. c  6. b  7. d

8. a 9. d 10. a

11. c  Note : Heavy water– Deuterium oxide (D₂O) is called heavy water. It’s molecular weight is 20 and boiling paint is 101.5⁰C and melting point is 3.8⁰C.

References

• Sthapit, M.K., Pradhananga, R.R., Foundations of Chemistry, Vol 1 and 2, Fourth edition, Taleju Prakashan, 2005.
• https://www.dictionary.com/browse/isobar
• https://www.angelo.edu/faculty/kboudrea/periodic/structure_mass.htm

The post Atom : Atomic number, Mass number, Isotopes, Isobars and Isotones appeared first on Online Chemistry notes.

The short name of the element is called symbol. Symbol of an element represents the specific element and also represents one atom of that element.

For example: – Symbol ‘K’ represents the element potassium and also represents one atom of potassium.

Some elements derived from the Latin name :-

Valency

Valency of an atom is the number of electrons that are lost or gained or shared by an atom during any chemical reaction (combination) in order to acquire stable configuration. Eg.

Variable Valency

Certain elements show more than one valency which is called variable valency. Most of the d- block elements (transition metals) shows variable valency.

The compound in which the metal has higher valency is called ‘-ic’ compound while the compound in which metal has lower valency is called an ‘-ous’ compound. Eg.

Certain elements can have more than two valencies. For example, valency of S in H₂S, SO₂ and SO₃ is 2, 4 and 6 respectively.

Radicals (Ions)

Most of the inorganic compounds are made up of two oppositely charged units called radicals or ions. Eg.

Radicals (ions) are atoms or group of atoms which carry positive or negative charge and behave as a single unit during a chemical reaction.

Depending upon the nature of charge, radicals are of two types:-

 1. Basic radical or electropositive radical or cation :

The radicals containing positive charge and derived from base are called basic radicals. Eg.

hydrogen(H⁺), calcium( Ca⁺⁺), aluminium(Al⁺⁺⁺), ammonium(NH₄⁺), etc.

2. Acidic radical or electronegative radical or anion :

The radicals containing negative charge and derived from acid are called acidic radicals. Eg.

Chloride (Cl^–), oxide(O^{– –}), sulphate(SO₄^{– –}), etc.

Some important electropositive radicals :

• Sodium = Na⁺
• Calcium = Ca⁺⁺
• Zinc = Zn⁺⁺
• Ammonium = NH₄⁺
• Aluminium = Al⁺⁺⁺
• Silicon = Si⁴⁺
• Cuprous = Cu⁺
• Cupric = Cu⁺⁺
• Mercurous = Hg⁺
• Mercuric = Hg⁺⁺
• Aurous = Au⁺
• Auric = Au³⁺
• Ferrous = Fe⁺⁺
• Ferric = Fe³⁺

Some important electronegative radicals :

• Chloride = Cl ^–
• Oxide = O ^{– –}
• Hydroxide = OH ^–
• Sulphide = S ^{– –}
• Sulphate = SO₄ ^{– –}
• Sulphite = SO₃ ^{– –}
• Thiosulphate = S₂O₃ ^{– –}
• Bisulphate = HSO₄^–
• Bisulphite = HSO₃^–
• Nitride = N ^3–
• Nitrate = NO₃^–
• Nitrite = NO₂^–
• Phosphate = PO₄^3-
• Carbonate = CO₃^{– –}
• Bicarbonate = HCO₃^–
• Manganate = MnO₄^{– –}
• Permanganate = MnO₄^–
• Dichromate = Cr₂O₇^{– –}
• Chromate = CrO₄^{– –}
• Cyanide = CN^–
• Cyanate = CNO^–
• Ferrocyanide = [Fe(CN)₆]^4-
• Ferricyanide = [Fe(CN)₆]^3-

Molecular formula

The symbolic representation of a molecule that shows actual number of atoms present in the molecule is called molecular formula.

For example: – HNO₃ represents one molecule of Nitric acid.

Writing molecular formula :

Simply apply criss-cross rule.

Eg. i. Sodium sulphate

ii. Ammonium carbonate

iii. Calcium ferrocyanide

Significance of molecular formula :

Qualitative significance :

1. Qualitatively formula of molecule represents the name of the substance. For examples: – CaCO₃ represent calcium carbonate.
2. It also tells the type of elements present in that molecule. For example, CaCO₃ contains the element calcium, carbon and oxygen.

Quantitative significance :

Molecular formula of H₂SO₄ represents its following quantitative significance:-

1. One molecule of the sulphuric acid.
2. One molecule of sulphuric acid contains two atoms of hydrogen, one atom of sulphur and four atoms of oxygen.
3. Molecular weight of the substance is obtained by adding the atomic weight of all the atoms present. Hence, molecular weight of sulphuric acid (H₂SO₄) is 98.
4. The relative weight of the elements presents in the substance is given by the molecular formula. In sulphuric acid molecule, weight ratio of hydrogen, sulphur and oxygen is 2:32:64 or 1:16:32 respectively.

Empirical formula

A formula that gives the simplest whole number ratio of atoms in a compound is called empirical formula.

For example: – Molecular formula of ethane is C₂H₆ and its empirical formula is CH₃. Similarly, the molecular formula of glucose is C₆H₁₂O₆ and its empirical formula is CH₂O.

Name and chemical formula of some common compounds

Questions and their answers 

1. Valency of N in N₂O₃ is :

a. 1    b. 2

c. 3    d. 4

2. Molecular formula of ferric ferrocyanide is :

a. Fe[Fe(CN)₆]₃    b. Fe₄[Fe(CN)₆]₃

c. Fe₂[Fe(CN)₆]₃   d. Fe₄[Fe(CN)₆]

3. Molecular formula of ‘laughing gas’ is :

a. NO₂     b. NO

c. N₂O₃    d. N₂O

4. Molecular formula of bleaching powder is :

a. CaSO₄     b. CaOCl₂

c. CuSO₄    d. CHCl₃

5. Symbol of element ‘Tin’ is :

a. Sn    b. Sb

c. Au    d. Ti

6. C₆H₁₂O₆ is molecular formula of :

a. Glucose      b. Fructose

c. Both ‘a’ and ‘b’    d.None of above.

7. Ratio of hydrogen, sulphur and oxygen by weight in a molecule of sulphuric acid is:

a. 1:32:64    b. 2:16:64

c. 2:16:32    d.1:16:32

8. Identify the valency of ‘N’ in N₂O, NO, NO₂ and N₂O₅.

9. Write the molecular formula of

1. Cupric chloride
2. Aluminium sulphate
3. Zinc nitrate
4. Ferric phosphate
5. Ammonium carbonate
6. Sodium phosphate
7. Potassium ferrocyanide
8. Potassium dichromate
9. Potassium ferricyanide
10. Ammonium cyanate

Answer..

1. – c       2. – b    3.- d(nitrous oxide)

4.- b    5.- a     6.- c    7.– d

8.- 1, 2, 4 and 5

9.- a) CuCl₂     b) Al₂(SO₄)₃

c) Zn(NO₃)₂    d) FePO₄

e) (NH₄)₂CO₃     f) Na₃PO₄

g) K₄[Fe(CN)₆]   h) K₂Cr₂O₇

i) K₃[Fe(CN)₆]     j) NH₄CNO

References

• Sthapit, M.K., Pradhananga, R.R., Foundations of Chemistry, Vol 1 and 2, Fourth edition, Taleju Prakashan, 2005.
• Negi, A.S., Anand, S.C., A Text Book of Physical Chemistry, Seventh Edition, New Age International Pvt. Ltd. Publishers, 1999.
• https://chem.libretexts.org/Courses/University_of_British_Columbia/CHEM_100%3A_Foundations_of_Chemistry/06%3A_Chemical_Composition/6.9%3A_Calculating_Molecular_Formulas_for_Compounds
• https://www.bbc.co.uk/bitesize/guides/zqrxsbk/revision/2
• https://www.quora.com/What-is-valency-in-chemistry

The post Symbol of elements, Variable valency, Radicals(ions) and Molecular formula appeared first on Online Chemistry notes.

Around 1780 chemists begin to distinguish chemical substances into two categories on the basis of source from which they were derived.

1. Organic compounds : Obtained from organism( i.e. plants or animals). Eg. glucose, protein, nucleic acid, etc.
2. Inorganic compounds : Prepared from non-living sources ( minerals). Eg. NaCl, SiO₂, etc.

In 1815, Berzelius (Swedish chemist) proposed vital force theory.

Vital force theory

According to the vital force theory, a vital/mysterious force present inside the living organism was responsible for the formation of organic compounds, so organic compounds can not be prepared in lab.

• Later, in 1828, German chemist, Friedrich Wohler obtained urea accidently in lab from ammonium cyanate ( by rearrangement).
• Wohler was in fact preparing ammonium cyanate by heating potassium cyanate and ammonium chloride.

• This was a milestone in organic chemistry.
• This preparation broke down the old concept i.e. vital force theory .
• The second organic compound synthesized in lab was acetic acid by Kolbe from it’s respective elements C, H and O.
• Third organic compound synthesized in lab was methane ( marsh gas) by Berthelot.
• Now more than 5 million organic compounds are known and hundreds of new organic compounds are being discovered daily.

Organic chemistry covers broad area :

Petrol, diesel, medicines, clothes, colours, DNA, enzymes, etc. are nothing but are organic compounds.

So, it is compulsory to know about organic chemistry to understand how does petrol/diesel produces energy, how does a medicine works, etc.

Modern definition of organic compounds

Organic compounds are defined as the hydrocarbons ( compounds containing carbon and hydrogen) and their derivatives in which covalently bonded carbon is an essential constituent.

Organic chemistry

Organic chemistry is the branch of chemistry which deals with the study of hydrocarbons and their derivatives.

Differences between organic and inorganic compounds

Some of the unique properties of carbon

(Reasons for separate study of organic compounds) :

 1. Tetracovalency of carbon :

The ground state electronic configuration of carbon is :

But it undergoes excited state during combination with other elements. In excited state, one of the paired electrons from 2s orbital gets promoted to vacant 2p_z orbital to form four unpaired electrons in the valence shell.

This atom acquires octet by sharing four electrons with other atoms during compound formation and thus shows tetracovalency. Eg.

 2. Catenation property :

The process of forming covalent bonds with atoms of the same element to give macromolecules or polymers ( long-chain) is called catenation property. It is one of the remarkable properties of carbon atom by which carbon atoms can link with each other to form either linear (straight) chain or branched chain or cyclic compounds. Eg.

 3. Capacity to form multiple bonds :

Carbon atom can form double or triple bonds with other elements like carbon, nitrogen, oxygen, etc. Eg.

The above mentioned unique properties of carbon have been able to form large number of organic compounds.

See the Classification of Organic compounds……

Objective questions and their answers

1) Vital force theory was first discarded by:

a. Berzelius   b. Wohler

c. Kolbe          d. Berthelot

2) The first organic compound synthesized in lab was

a. Urea              b. Methane

c. Acetic acid   d. Ethane

3) The first organic compound synthesized in lab from its elements was 

a. Urea              b. Methane

c. Acetic acid   d. Ethane

4 ) NH₄CNO → NH₂CONH₂ . This reaction is:

a. Addition reaction

b. Elimination reaction

c. Rearrangement reaction

d. Substitution reaction

5) ……… is often called as ‘king of element’.

a. H    b. C

c. N    d. O

6) Carbon always forms …….covalent bonds.

a. 2    b. 3

c. 4    d. 5

7) Organic compounds are very large in number. This is due to:

a. Small size of carbon

b. Catenation property of carbon

c. Valency of carbon

d. All of the above

Answer … 1-b( discarded- dismissed, rejected), 2-a, 3-c, 4-c, 5-b, 6-c, 7-b.

References

• Morrison, R.T. , Boyd, R.N., Organic Chemistry, Sixth edition, Prentice-Hall of India Pvt. Ltd., 2008.
• Sthapit, M.K., Pradhananga, R.R., Foundations of Chemistry, Vol 1 and 2, Fourth edition, Taleju Prakashan, 2005.
• https://www.sciencedirect.com/topics/earth-and-planetary-sciences/organic-chemistry
• https://chemistry.stanford.edu/research/research-areas/organic-chemistry

The post Introduction and history of Organic Chemistry; Unique properties of carbon appeared first on Online Chemistry notes.

The branch of science which deals with the study of composition, structure, properties and change of matter is called chemistry.

Every material in existence is made up of matter - even our own bodies. Chemistry is involved in everything we do, from growing and cooking food to cleaning our homes and bodies to launching a space shuttle. Chemistry is one of the physical sciences that help us to describe and explain our world.

Many people think of chemists as being white-coated scientists mixing strange liquids in a laboratory, but the truth is we are all chemists. Understanding basic chemistry concepts is important for almost every profession. Chemistry is part of everything in our lives. 

Matter and it’s classification

Anything that occupies space and has weight and can be felt (by any one or more of our senses) is called matter.

Everything that is around us including the pen, book, pencil, air, all living beings are composed of matter. They all have mass and occupy space.

Classification of matter can be done by two ways – physical and chemical classification.

States of matter :

There are four states of matter in the universe: solid, liquid, gas and plasma. But, matter on earth exists mostly in three states: solid, liquid and gas.

Differences between solid, liquid and gas :

Generally, on heating – solid changes to liquid and liquid changes to gas. On the other hand, on cooling – gas changes to liquid and liquid changes to solid. Eg.

What is Plasma ?

At very high temperature(of stars), atoms lose their electrons. The mixture of electrons and nuclei forms plasma state of matter.

Like gases, plasma have not fixed shape and volume, and are less dense than solids or liquids. But unlike ordinary gases( which are neutral), plasmas are made up of electrically charged particles(ions and electrons). Plasma makes up the sun and other stars, and it is the most common state of matter in the universe as a whole.

Changes of Phase : freezing, melting, condensation, vaporization, sublimation and deposition

A phase is a distinctive form of a substance, and matter can change among the phases. It may take extreme temperature, pressure or energy, but all matter can be changed.

There are six distinct changes of phase which happens to different substances at different temperatures. The six changes are:

• Freezing – the substance changes from a liquid to a solid.
• Melting – the substance changes back from the solid to the liquid.
• Condensation – the substance changes from a gas to a liquid.
• Vaporization – the substance changes from a liquid to a gas.
• Sublimation – the substance changes directly from a solid to a gas without going through the liquid phase.
• Deposition: the substance changes directly from a gas to a solid without going through the liquid phase.
• Ionization – Ions are formed by gain or loss of electrons from an atom or molecule.

Elements

An element is a simplest form of a pure substance that can neither be decomposed into nor built up from simpler substances by ordinary physical or chemical process.

An element is a substance that is made entirely from one type of atom. For example, the element hydrogen is made from atoms containing a single proton and a single electron. If you change the number of protons an atom has, you change the type of element it is.

There are total 118 elements with their atomic number from 1 to 118. Out of which 92 ( approximation) occur naturally while the rest are prepared artificially in the laboratories.

Elements are further classified as metals, non- metals and metalloids. Out of the 118 elements of the periodic table, 84 are metals, 7 are metalloids and rests of them are non-metals.

1. Metals : Metals are generally solids (except mercury, which is the only metal which exists in liquid state in room temperature). They are good conductors of heat and electricity, and are malleable (they can be hammered into sheets) and ductile (they can be drawn into wire). Iron, Aluminum, Copper, Silver and Gold are common example of metal

2. Non-metals : Non-metals are generally found in gaseous state but iodine is found in solid state and bromine is the found in liquid state. They are usually poor conductor of heat and electricity and are not malleable and ductile. Examples of non-metal are carbon, hydrogen, oxygen, nitrogen, sulphur, etc.

3. Metalloids : The elements which possess both the characteristics of metals as well as non-metals are called metalloids. In their physical properties, they are more like the nonmetals, but under certain circumstances, several of them can be made to conduct electricity. Some examples of metalloid are Silicon, Arsenic, Bismuth, Antimony, etc.

Compound

A compound is a pure substance formed by the chemical union of two or more elements in a fixed proportional by weight. For example:- water is a compound of hydrogen and oxygen combined together in a fixed proportional 1:8 by weight.

The properties of compound are entirely different from it’s constituents. For example, hydrogen burns, oxygen supports burning but water(containing hydrogen and oxygen) neither burns nor supports the burning.

Mixture

Anything obtained by mixing two or more substances (elements or compounds) in any proportion so that their components do not lose their identity is called mixture. Mixture can be separated by different method depending upon the nature of mixing components. Some of the methods are filtration, sublimation, evaporation, distillation, crystallization, etc.

Mixture is of two types:-

1. Homogenous mixture:-

The mixture in which the components mixed are uniformly distributed throughout the mixture is called homogeneous mixture. The mixing components cannot be seen. Homogeneous mixtures are also called as solutions

Example: – alcohol in water, air, petrol, sugar solution, etc.

2. Heterogeneous mixture:-

The mixture in which the components mixed are not uniformly distributed in the mixture is called heterogeneous mixture. The mixing components can be seen through our naked. Examples:- Mixture of oil and water, sand and water, iron and wood dust, etc.

Differences between compound and mixture :

Atoms and Molecules

Atom :

An atom is the smallest particle of an element which can take part in chemical reaction. Atom consists of three fundamental particles like proton, neutron and electron. Atoms of same elements are similar in properties whereas atoms of different elements are different in properties. Example:- ‘H’ represent the atom of hydrogen.

Atoms may or may not have independent existence. Atoms of inert gases like helium, neon, argon, etc. have independent existence whereas atoms of oxygen, nitrogen, etc. do not have independent existence. Atoms combine with each other to form stable molecules (like O₂).

Molecules :

Molecule is the smallest unit of an element or a compound which can exist in free state in nature and possess all the properties of the element or compound.

→ Molecules containing atoms of same element are called homoatomic molecules. Eg. H₂, N₂, P₄, O₂, O₃, S₈, etc.

→ Molecules containing atoms of different elements are called heteroatomic molecules. Eg. HCl, CO₂, NH₃, CH₄, PCl₅, etc.

Depending on the number of atoms presenet, a molecule is called monoatomic, diatomic and polyatomic molecule. Eg.

• Monoatomic molecule – He, Ne, Ar, Kr, Xe and Rn.
• Diatomic molecule – H₂, NaCl, O₂, HCl, etc.
• Polyatomic molecule – P₄, NH₃, H₂SO₄, etc.

Difference between compound and molecule :

• A compound is a substance that is composed from two or more different elements. Eg. Water (H₂O), table salt (NaCl), carbon dioxide (CO₂), methane (CH₄), etc .
• A molecule is a group of two or more atoms held together by chemical bonds. Eg. H₂ , NaCl, N₂, etc.
• All compounds are molecules but all molecules are not compound. Things like nitrogen gas (N₂), oxygen(O₂), etc.are molecules but not compounds since they only contain one kind of element.

Objective questions

Q 1) Air is an example of :

a. compound.

b. homogeneous mixture.

c. heterogeneous mixture.

d. molecule.

Q 2) On heating, solid camphor directly changes to gas, this process is called :

a. evaporation

b. melting

c. deposition

d. sublimation.

Q 3) Proton and neutron is combinely called :

a. electron.

b. atom.

c. nucleon

d. element.

Q 4) Plasma is formed by :

a. ionization of gas.

b. condensation of gas.

c. melting of solid.

d. deposition of gas.

Q 5) ‘He’ indicates :

a. An atom of helium

b. A molecule of helium

c. both a and b

d. An ion of helium.

Q6) Which one of the following is not a pure substance?

a. Diamond

b. Gold

c. Air

d. Ammonia

Q7) The chemical symbol for tungsten is

a. Tg

b. W

c. Tu

d. Hg

8)  Which of the following elements show the characteristics of both metals and non-metals?

a. Si

b. Br

c. Ti

d. K

9) HCl is an example of

a. Diatomic molecule.

b. Heteroatomic molecule

c. Homoatomic molecule

d. Both ‘a’ and ‘b’

10) The most common state of matter in the universe is

a. Solid

b. Liquid

c. Gas

d. Plasma

Answer 

1- b, 2-d, 3-c, 4-a, 5-c, 6-c, 7-b, 8-a, 9-d, 10-d

References 

• Sthapit, M.K., Pradhananga, R.R., Foundations of Chemistry, Vol 1 and 2, Fourth edition, Taleju Prakashan, 2005.
• https://iupac.org/
• https://www.chem.purdue.edu/gchelp/atoms/elements.html
• https://www.bbc.co.uk/bitesize/guides/zgdyh39/revision/1

The post Basic chemistry concepts – Matter, Element, Atom, Molecule, Compound and Mixture. appeared first on Online Chemistry notes.