The noble gas that does NOT occur in the atmosphere is:

CONCEPT:

Noble Gases in the Atmosphere

• Noble gases are a group of chemical elements with similar properties. They are all odorless, colorless, monatomic gases with very low chemical reactivity.
• The noble gases that occur naturally in the atmosphere include helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe).

EXPLANATION:

• Radon (Ra) is a noble gas but it does NOT occur in the atmosphere in significant amounts because it is radioactive and has a short half-life.
• The other noble gases listed (helium (He), neon (Ne), and krypton (Kr)) do occur in the atmosphere.

Therefore, the noble gas that does NOT occur in the atmosphere is Radon (Ra).