Write the chemical formula of Potassium dichromate in a step-by-step manner.

Step 1 — Write each symbol with its valency

$\text{K}^{1+} \phantom{\nearrow} \text{Cr}$2\text{O}7^{2-}

Step 2 — Interchange the valencies

$\overset{\phantom{2}{1}}{\text{K}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{Cr}$2\text{O}7} \Rightarrow \underset{\phantom{1}{2}}{\text{K}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{Cr}2\text{O}7} \\[0.5em]

Step 3 — Write the interchanged number & hence the formula

Therefore, we get

Formula of Potassium dichromate : $\bold{K$2Cr2O_7}