* Helium is an inert gas: Helium is in group 18 (VIIIa) of the periodic table, the noble gases. These elements have a full outer shell of electrons, making them very stable and unreactive.
* Phosphorus is a nonmetal: While phosphorus can be quite reactive, it primarily reacts with other nonmetals or electropositive metals.
In essence, helium's stability and lack of a strong tendency to gain or lose electrons prevent it from reacting with phosphorus.
