Noble gases

Noble gases are unreactive gases whose particles are individual atoms, rather than molecules made of bonded atoms. They are therefore described as monatomic.

Noble gas atoms have full outer electron shells. A helium atom has two electrons, filling its only shell. Atoms of the other noble gases have eight electrons in their outer shells.

A full outer shell is a stable electron arrangement. Noble gas atoms therefore have little tendency to gain, lose or share electrons, so they rarely form chemical bonds. This explains both their low reactivity and why they usually exist as separate atoms.