Here's why:

* Elements are pure substances made up of only one type of atom (like hydrogen (H) or phosphorus (P)).

* Molecules are formed when two or more atoms bond together, regardless of whether they are the same type of atom or different.

PH3 is made of two different types of atoms: phosphorus (P) and hydrogen (H), bonded together. Therefore, it's a molecule, specifically the molecule of phosphine.