Here's why:

* Ammonium ion (NH₄⁺) is a cation: It has a positive charge.

* Valency represents the combining power of an element or radical: In this case, it shows how many electrons ammonium needs to lose or gain to achieve a stable configuration.

* Ammonium has a +1 charge: This means it has lost one electron and has one extra proton, giving it a net positive charge.

Therefore, the valency of ammonium is +1.