Write the chemical formula of Iron [III] sulphide in a step-by-step manner.

Step 1 — Write each symbol with its valency

$\text{Fe}^{3+} \phantom{\nearrow} \text{S}^{2-}$

Step 2 — Interchange the valencies

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

Step 3 — Write the interchanged number & hence the formula

Therefore, we get

Formula of Iron [III] sulphide : $\bold{Fe$2S3}