Monatomic gas
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. All chemical elements will be monatomic in the gas phase at sufficiently high temperatures.
Noble gases
The only chemical elements which are stable single atom molecules at standard temperature and pressure (STP), are the noble gases. These are helium, neon, argon, krypton, xenon and radon. The heavier noble gases can form chemical compounds, but the lighter ones are non-reactive or inert. For example, helium, being the simplest noble gas, has only two electrons, thus meaning it is satisfied with a complete outer shell, making it relatively non-reactive, hence it is a noble gas.
The noble gases, when grouped together with the homonuclear diatomic gases, such as nitrogen (N2), are called "elemental gases" or "molecular gases" to distinguish them from molecules that are also chemical compounds.
