* Compounds are formed when two or more different elements chemically combine in a fixed ratio.

Let's analyze each option:

* CH4 (Methane): Carbon (C) and Hydrogen (H) combined.

* H2O (Water): Hydrogen (H) and Oxygen (O) combined.

* NH4 (Ammonium): Nitrogen (N) and Hydrogen (H) combined.

* N2 (Nitrogen gas): This is a diatomic molecule, meaning it consists of two atoms of the same element (Nitrogen) bonded together. It's not a compound because it's composed of only one type of element.

Therefore, N2 is not a compound.