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 unreactive.

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.

Other elements

Monatomic hydrogen comprises about 75% of the elemental mass of the universe.

Chemists noticed very long ago that when some gases (e.g. hydrogen, oxygen, chlorine) are being created e.g. during electrolysis or in reactions of metals with acids (hydrogen), they are radically more reactive than normally. This state of gases has been named "in statu nascendi" (Latin, pron. nahstzendee), meaning "in the state of being born". That aggressive reactivity can be used to intensify some chemical reactions. The source of that high reactivity is the fact that such gases remain monoatomic radicals for some time; the presence of some poisons e.g. arsenic can prolong that monoatomic state

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:

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.