In physics and chemistry, "monatomic" is a combination of the words "mono" and "atomic", and means "single atom". It is usually applied to : a monatomic gas is a gas in which atoms are not bound to each other. Examples at standard conditions of temperature and pressure include all the (helium, neon, argon, krypton, xenon, and radon), though all chemical elements will be monatomic in the gas phase at sufficiently high temperature (or very low pressure). The Thermodynamics behavior of a monatomic gas is much simpler when compared to polyatomic gases because it is free of any rotational or vibrational energy.
When grouped together with the homonuclear diatomic gases such as nitrogen (N2), the noble gases are called "elemental gases" to distinguish them from molecules that are also chemical compounds.
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: Heat Capacity of an Ideal Gas Heat Capacity of Ideal Gases Lecture 3: Thermodynamics of Ideal Gases & Calorimetry, p. 2
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