Monatomic

In physics and chemistry, monatomic is a combination of the words "mono" and "atomic," and means "single atom." It is usually applied to gases: a monatomic gas is one in which atoms are not bound to each other.

Under Standard Temperature and Pressure (STP), all of the noble gases are monatomic. To be a monatomic gas, an element usually has to have zero valence (as for the noble gases); or be a normally diatomic gas under extremely low pressure, as is the case with oxygen in Earth's upper atmosphere.