* Argon (Ar) is a noble gas. Noble gases have a full outer shell of electrons, making them extremely stable and unreactive. They rarely form compounds.

* Phosphorus (P) is a nonmetal. It has five valence electrons, meaning it needs to gain three electrons to achieve a stable octet. This makes it highly reactive, especially with oxygen, forming compounds like phosphorus pentoxide (P₂O₅).

In summary: Argon is very unreactive due to its full outer shell, while phosphorus is quite reactive due to its incomplete outer shell.