Examples at standard conditions of temperature and pressure include all the noble gases (helium, neon, argon, krypton, xenon, and radon), though all chemical elements will be monatomic in the gas phase at sufficiently high temperature (or very low pressure).
The thermodynamic behavior of a monatomic gas is much simpler when compared to polyatomic gases because it is free of any rotational or vibrational energy.
[1] The only chemical elements that are stable single atoms (so they are not molecules) at standard temperature and pressure (STP) are the noble gases.
The only possible motion of an atom in a monatomic gas is translation (electronic excitation is not important at room temperature).
Thus by the equipartition theorem, the kinetic energy of a single atom of a monatomic gas at thermodynamic temperature T is given by