noble gases

(Also called rare gases, inert gases.) The elements helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) that make up the eighth period of the periodic table.

Because of their completely filled atomic orbitals, these gases are extremely nonreactive, and build up to measurable concentrations in the atmosphere.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.