NOVA Hunting the Elements Practice Test

Reactivity is driven by whether an atom can reach a stable electron arrangement by gaining, losing, or sharing electrons. Noble gases have full valence electron shells (helium with a complete 1s shell, the others with a complete ns2 np6 outer shell). That complete outer shell is already a low-energy, stable arrangement, so there’s little motivation for them to rearrange their electrons or form bonds. In other words, the energy cost to remove or add electrons is high, so they stay largely inert under normal conditions.

Notes on the other choices: low atomic mass doesn’t determine reactivity; noble gases are mostly monoatomic, not diatomic; and they aren’t metals, so those properties don’t explain their lack of reactivity.