Write down the formulae of

(i) Sodium oxide

(ii) Aluminium chloride

(iii) Sodium sulphide [3 MARKS]​

Criss -cross method: In this method, the numerical value of each of the ion charges is crossed over to become the subscript of the other ion.
1. Sodium oxide:
Symbol: Na O
Valency: 1 2
Criss-crossing the valencies we get the formula as $N{a}_{2}O$

2. Aluminium chloride:
Symbol: Al Cl
Valency:3 1
Criss-crossing the valencies we get the formula as $AlC{l}_3$

3. Sodium sulphide:

Symbol: Na S
Valency: 1 2
Criss-crossing the valencies we get the formula as $N{a}_{2}S$