Question

What occurs when Cr^3+ ions are reduced to Cr^2+ ions?

a) electrons are lost and the oxidation number of chromium increases

b) electrons are lost and the oxidation number of chromium decreases

c) electrons are gained and the oxidation number of chromium increases

d) electrons are gained and the oxidation number of chromium decreases

Answer 1

D:

When electrons are gained, the charge of the atom decreases.

When you are given an atom with a charge, the oxidation of that atom is the charge. So by going from a Cr^3+ (Oxidation Number = 3) to a Cr^2+ (Oxidation Number = 2), the Oxidation Number thus decreases.