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Monatomic gas
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In and , "monatomic" is a combination of the words "mono" and "atomic", and means "single ". It is usually applied to : a monatomic gas is one in which atoms are not bound to each other. Examples at standard conditions include the noble gases argon, krypton, and xenon, though all chemical elements will be monatomic in the gas phase at sufficiently high temperatures. The behavior of a monatomic gas is extremely simple when compared to polyatomic gases because it is free of any rotational or vibrational energy.


Noble gases
The only chemical elements that are stable single at standard temperature and pressure (STP) are the . These are , , , , , and . Noble gases have a full outer valence shell making them rather non-reactive species. While these elements have been described historically as completely inert, chemical compounds have been synthesized with all but neon and helium.

When grouped together with the gases such as (N2), the noble gases are called "elemental gases" or "molecular gases" to distinguish them from molecules that are also chemical compounds.


Thermodynamic properties
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 of a single atom of a monatomic gas at thermodynamic temperature T is given by \frac{3}{2}k_bT, where kb is Boltzmann's constant. One mole of atoms contains an (\mathcal{N}_a) of atoms, so that the energy of one mole of atoms of a monoatomic gas is \frac{3}{2} k_b T \mathcal{N}_a = \frac{3}{2} RT , where R is the .

In an adiabatic process, monatomic gases have an idealised γ-factor ( Cp/ Cv) of 5/3, as opposed to 7/5 for ideal gases where rotation (but not vibration at room temperature) also contributes. Also, for ideal monatomic gases: Heat Capacity of an Ideal Gas Heat Capacity of Ideal Gases Lecture 3: Thermodynamics of Ideal Gases & Calorimetry, p. 2

the molar at constant pressure ( Cp) is 5/2  R = 20.8 J K−1 mol−1 (4.97  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).

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