When an atom in a reactant gains electrons, what happens to its oxidation number?
its oxidation number decreases.
its oxidation number doubles.
its oxidation number increases.
its oxidation number increases by one.
its oxidation number stays the same.

When an atom in a reactant gains electrons Its oxidation number decreases.

Explanation:

Oxidation number decreases when an atom gains an electrons. This type of reaction is called Reduction reaction. In this number of electrons increases in an atom.

For example:

Here iron's oxidation state change to from+3 to +2 by gaining one electron.