→ An ion may be defined as an atom or group of atoms having positive or negative charge.

→ Some positively charged ions : Na+ , K+ , Ca2+ , Al3+

→ Some negatively charged ions : Cl- (chloride ion), S2- (sulphide ion), OH-(hydroxide ion),

SO42- (sulphate ion)

• We can classify ions in two types:

(i) Simple ions

Mg2+ (Magnesium ion)

Na+(Sodium ion)

Cl-(Chloride ion)

Al3+ (Aluminium ion)

(ii) Compound ions

NH4+ (Ammonium ion)

CO32- (Carbonate ion)

SO42- (Sulphate ion)

OH- (Hydroxide ion)

• Chemical Formulae of Ionic Compounds (Polyatomic)

Mole Concept

→ A group of 6.022×1023 particles (atoms, molecules or ions) of a substance is called a mole of


→ 1 mole of atoms = 6.022×1023 atoms

→ 1 mole of molecules = 6.022 × 1023 molecules

Example, 1 mole of oxygen = 6.022×1023 oxygen atoms

Note: 6.022×1023 is Avogadro Number (L).

→1 mole of atoms of an element has a mass equal to gram atomic mass of

the element.

Molar Mass

→ The molar mass of a substance is the mass of 1 mole of that substance.

→ It is equal to the 6.022×1023 atoms of that element/substance.


(a) Atomic mass of hydrogen (H) is 1 u. Its molar mass is 1 g/mol.

(b) Atomic mass of nitrogen is 14 u. So, molar mass of nitrogen (N) is 14 g/mol.

(c) Molar mass of S8 = Mass of S×8 = 32×8 = 256 g/mol

(d) Molar mass of HCl = Mass of H + Mass of Cl = 1 = 35.5 = 36.5 g/mol

Important Formulae -