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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.
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 non-reactive or inert. For example, helium, the simplest noble gas, has a complete outer valence shell. This makes it quite unreactive and unlikely to form compounds.
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:
- the molar heat capacity at constant pressure (Cp) 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 (Cv) is 3/2 R = 12.5 J K−1 mol−1 (2.98 cal K−1 mol−1);
where R is the gas constant.