The chemical formula NH₄ represents the ammonium ion, which is a single ion.

Here's why:

* NH₄ represents a polyatomic ion, which is a group of atoms bonded together that carry a net charge.

* In this case, the ammonium ion (NH₄⁺) consists of one nitrogen atom (N) and four hydrogen atoms (H) bonded together, with a +1 charge.

Therefore, while there are 5 atoms in the ammonium ion, it is considered one single entity with a positive charge, acting as a single ion.