Ammonium (NH₄⁺) is neither a metal nor a nonmetal. It is a polyatomic ion, which means it is a group of atoms that act as a single unit with an overall charge.

Here's why:

* Metals are typically shiny, malleable, ductile, and good conductors of heat and electricity. They tend to lose electrons to form positive ions.

* Nonmetals are generally dull, brittle, poor conductors of heat and electricity. They tend to gain electrons to form negative ions.

Ammonium does not fit into either category. It is a positively charged ion (cation) formed by the combination of one nitrogen atom and four hydrogen atoms.