Explain the term – 'valency'. State giving reasons the valency of –

(a) chlorine in hydrogen chloride

(b) sulphate in sulphuric acid

(c) nitrogen in ammonia

(d) magnesium in magnesium oxide

Valency of an element or radical is the number of hydrogen atoms which can combine with or displace one atom of the element or radical forming a compound.

(a) One atom of hydrogen combines with one atom of chlorine to form HCl, hence the valency of chlorine in hydrogen chloride is 1^-.

(b) Two atoms of hydrogen combine with one radical of sulphate to form sulphuric acid (H₂SO₄), hence the valency of sulphate in sulphuric acid is 2^-.

(c) Three atoms of hydrogen combine with one atom of nitrogen to form ammonia (NH₃) hence the valency of nitrogen in ammonia is 3^-.

(d) We know that valency of oxygen is 2^-. Hence, in magnesium oxide [MgO], valency of magnesium is 2⁺.