Here's why:

* Ammonium is formed when ammonia (NH₃) gains a hydrogen ion (H⁺).

* Ammonia has a lone pair of electrons on the nitrogen atom, which can accept a proton.

* When the proton is accepted, the nitrogen atom gains a positive charge, forming the ammonium ion (NH₄⁺).

So, the ammonium ion has the formula NH₄⁺.