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. Noble gases have a full outer valence shell making them rather non-reactive species. While these elements have been described historically as completely inert, chemical compounds have been synthesized with all but neon and helium.
When grouped together with the homonuclear diatomic gases such as nitrogen (N2), the noble gases are called "elemental gases" or "molecular gases" to distinguish them from molecules that are also chemical compounds.
The motion of a monatomic gas is translation (electronic excitation is not important at room temperature). Thus in an adiabatic process, monatomic gases have an idealised γ-factor (Cp/Cv) of 5/3, as opposed to 7/5 for ideal diatomic gases where rotation (but not vibration at room temperature) also contributes. Also, for ideal monatomic gases:
where R is the gas constant.