Here's why:

* Nitrogen's electron configuration: Nitrogen has 5 valence electrons. To achieve a stable octet, it wants to gain 3 more electrons.

* Formation of negative ions: When nitrogen gains 3 electrons, it forms a nitride ion (N³⁻), resulting in an oxidation number of -3.

* Examples: This oxidation state is seen in compounds like ammonia (NH₃) and nitrides (e.g., Mg₃N₂).

While -3 is the most common, nitrogen can exhibit other oxidation states ranging from -3 to +5, depending on the chemical environment.