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.

At standard temperature and pressure (STP), all of the noble gases are monatomic. These are helium, neon, argon, krypton, xenon and radon. The heavier noble gases can form compounds, but the lighter ones are unreactive. All elements will be monatomic in the gas phase at sufficiently high temperatures.

The only mode of 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 (C_p/C_v) 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:

the molar heat capacity at constant pressure (C_p) is 5/2 R = 20.8 J K^-1 mol^-1 (4.97 cal K^-1 mol^-1);

the molar heat capacity at constant volume (C_v) is 3/2 R = 12.5 J K^-1 mol^-1 (2.98 cal K^-1 mol^-1);

where R is the gas constant.