When an atom in a reactant loses electrons, what happens to its oxidation number?

Its oxidation number increases. When an atom loses electrons, there are now more protons then electrons so its oxidation number increases.

Explanation:

The oxidation product of an element signifies the hypothetical charge of an atom in a compound. It is hypothetical because, in the context of a compound, the elements may not absolutely be ionic. When the number of electrons associated with an atom changes, its oxidation number also changes. When an element loses an electron, its oxidation number rises.

false

because it does not have the potential to be entirely used up