1. Determine the initial oxidation number of chromium in CrO₄²⁻:
* Oxygen typically has an oxidation number of -2.
* The overall charge of the chromate ion (CrO₄²⁻) is -2.
* Let x be the oxidation number of chromium.
* Using the formula: x + 4(-2) = -2
* Solving for x, we get x = +6.
2. Determine the final oxidation number of chromium in Cr³⁺:
* The oxidation number of a monatomic ion is equal to its charge.
* Therefore, the oxidation number of chromium in Cr³⁺ is +3.
3. Calculate the change in oxidation number:
* Change in oxidation number = Final oxidation number - Initial oxidation number
* Change in oxidation number = +3 - +6 = -3
Therefore, the change in oxidation number of the chromium atom is -3. This means the chromium atom gained 3 electrons during the reaction.
