Symbol of elements, Variable valency, Radicals(ions) and Molecular formula

Symbol of elements

---

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.

Atomic number	Name of element	Symbol
1	Hydrogen	H
2	Helium	He
3	Lithium	Li
4	Beryllium	Be
5	Boron	B
6	Carbon	C
7	Nitrogen	N
8	Oxygen	O
9	Fluorine	F
10	Neon	Ne
11	Sodium	Na
12	Magnesium	Mg
13	Aluminium	Al
14	Silicon	Si
15	Phosphorus	P
16	Sulfur	S
17	Chlorine	Cl
18	Argon	Ar
19	Potassium	K
20	Calcium	Ca
21	Scandium	Sc
22	Titanium	Ti
23	Vanadium	V
24	Chromium	Cr
25	Manganese	Mn
26	Iron	Fe
27	Cobalt	Co
28	Nickel	Ni
29	Copper	Cu
30	Zinc	Zn

Some elements derived from the Latin name :-

Elements	Latin name	Symbol
Sodium Potassium Iron Copper Mercury Gold Silver Lead Tin Antimony Tungsten	Natrium Kalium Ferrum Cuprum Hydragyrum Aurum Argentum Plumbum Stannum Stibium Wolfram	Na K Fe Cu Hg Au Ag Pb Sn Sb W

---

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.

compound	element	Valency
NaCl	Na or Cl	1
MgO	Mg	2
AlCl3	Al	3
HCl	H or Cl	1
CH4	C	4

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.

Metal	Valency	Molecular formula	Name of compound	IUPAC name
Fe	2	FeO	Ferrous oxide	Iron(I) oxide
	3	Fe2O3	Ferric oxide	Iron(II) oxide
Cu	1	Cu2O	Cuprous oxide	Copper(I) oxide
	2	CuO	Cupric oxide	Copper(II) oxide
Hg	1	HgCl or Hg2Cl2	Mercurous chloride	Mercury(I) chloride
	2	HgCl2	Mercuric chloride	Mercury(II) chloride
Ag	1	AgCl	Argentous chloride	Silver(I) chloride
	3	AgCl3	Argentic chloride	Silver(III) chloride
Au	1	AuCl	Aurous chloride	Gold(I) chloride
	3	AuCl3	Auric chloride	Gold(III) chloride
Sn	2	SnCl2	Stannous chloride	Tin(II) chloride
	4	SnCl4	Stannic chloride	Tin(IV) chloride
Pb	2	PbCl2	Plumbous chloride	Lead(II) chloride
	4	PbCl4	Plumbic chloride	Lead(IV) chloride

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

---

General name	Chemical name	Formula
water	Dihydrogen monoxide	H2O
Common salt/ table salt/ rock salt	Sodium chloride	NaCl
Sand/Quartz	Silicon dioxide	SiO2
Marble/Lime Stone	Calcium carbonate	CaCO3
Lime/Quick lime	Calcium Oxide	CaO
Washing Soda	Sodium Carbonate	Na2CO3
Caustic Soda	Sodium Hydroxide	NaOH
Caustic Potash	Potassium Hydroxide	KOH
Baking Powder/backing soda	Sodium bicarbonate	NaHCO3
Paraffins	Alkane	CnH2n+2
Olefins	Alkene	CnH2n
Ethylene	Ethene	C2H4
Acetylene	Ethyne	C2H2
Black oxide of copper	Copper(II)oxide	CuO
Red oxide of copper	Copper(I)oxide	Cu2O
Bleaching Powder	Calcium oxychloride	CaOCl2
Blue vitriol	Copper sulphate pentahydrate	CuSO4.5H2O
White vitriol	Zinc sulphate heptahydrate	ZnSO7.7H2O
Green vitriol	Ferrous sulphate heptahydrate	FeSO7.7H2O
Plaster of Paris	Calcium sulphate semihydrate	CaSO4.1/2H2O or 2CaSO4.H2O
Calomel	Mercurous chloride	Hg2Cl2
Dry ice	Solid carbondioxide	CO2
Glucose	Aldohexose	C6H12O6
Laughing gas	Nitrous oxide	N2O
Tear gas/chloropicrin	Trichloronitromethane	CCl3.NO2
Sindur (Red lead)	Triplumbic tetraoxide	Pb3O4
Urea	Carbamide /amino methanamide	NH2CONH2
Vinegar	Acetic acid	CH3COOH
White gold	Platinum	Pt

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