Write down the formulae of

(i) sodium oxide

(ii) aluminium chloride

(iii) sodium sulphide

(iv) magnesium hydroxide

(i) Sodium oxide:

Step 1 — Write each symbol with its valency

$\text{Na}^{1+} \phantom{\nearrow} \text{O}^{2-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{O}} \Rightarrow \underset{\phantom{1}{2}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{O}} \\[0.5em]$

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Sodium oxide : $\bold{Na_2O}$

(ii) Aluminium chloride

Step 1 — Write each symbol with its valency

$\text{Al}^{3+} \phantom{\nearrow} \text{Cl}^{1-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{3}}{\text{Al}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{Cl}} \Rightarrow \underset{\phantom{1}{1}}{\text{Al}} \space {\searrow}\mathllap{\swarrow} \space \underset{3}{\text{Cl}} \\[0.5em]$

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Aluminium chloride : $\bold{AlCl_3}$

(iii) sodium sulphide

Step 1 — Write each symbol with its valency

$\text{Na}^{1+} \phantom{\nearrow} \text{S}^{2-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{S}} \Rightarrow \underset{\phantom{1}{2}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{S}} \\[0.5em]$

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Sodium sulphide : $\bold{Na_2S}$

(iv) Magnesium hydroxide

Step 1 — Write each symbol with its valency

$\text{Mg}^{2+} \phantom{\nearrow} \text{OH}^{1-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{2}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{OH}} \Rightarrow \underset{\phantom{1}{1}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{OH}} \\[0.5em]$

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Magnesium hydroxide : $\bold{Mg(OH)_2}$