Why does nitrogen have a charge of negative 3 when it participates in a chemical reaction?

Nitrogen does not tend to form ionic compounds as a N3- (Nitride) ion it is highly unstable but can exist in a crystal lattice in compounds such as Li3N (lithium nitride) it cannot exist in solution like a chloride ion. Nitrogen can found in any oxidation state however from +5 to -3 but it's bonds are covalent in nature and not an ionic compound.

I think the poster might be coming from a intro to bio or chemistry class, in relation to electrons and outer shells.

so.

Nitrogen

• 2 electrons in its inner shell
• 5 electrons in its outer shell

So it takes less energy for nitrogen to gain 3 electrons than it would to try and lose 5 ( to fill its outer shell/valence shell)