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

Step 1 — Write each symbol with its valency

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

Step 2 — Interchange the valencies

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

Step 3 — Write the interchanged number & hence the formula

Therefore, we get

Formula of Iron [III] chloride : $\bold{FeCl_3}$