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. The thermodynamic behavior of monatomic gas is extremely simple when compared to polyatomic gases because it is free of any rotational or vibrational energy.^[1]

Noble gases

The only chemical elements that 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.^[2] While these elements have been described historically as completely inert, chemical compounds have been synthesized with all but neon and helium.^[3]

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.

Other elements

Monatomic hydrogen comprises about 75% of the elemental mass of the universe.^[4]

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 (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:^[5][6][7]

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.

References