Here's why:

* Ammonium is a polyatomic ion composed of nitrogen and hydrogen.

* Nitrogen has a valence of 3, meaning it can form 3 bonds.

* Hydrogen has a valence of 1, meaning it can form 1 bond.

* To achieve a stable octet, nitrogen needs 5 electrons (it has 5 valence electrons), so it forms 4 bonds with hydrogen.

* The overall charge of the ion is +1 because nitrogen has gained an electron.

Therefore, the formula for ammonium ion is NH₄⁺.