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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


































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.


valency of carbon

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 = PO43-
  • 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

criss- cross rule to write molecular formula

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


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