:Monatomic gas

The only chemical elements that are stable single atoms (so they are not 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.{{Cite journal|last1=Laszlo|first1=Pierre|last2=Schrobilgen|first2=Gary J.|date=1988-04-01|title=Ein Pionier oder mehrere Pioniere? Die Entdeckung der Edelgas-Verbindungen|journal=Angewandte Chemie|language=en|volume=100|issue=4|pages=495–506|doi=10.1002/ange.19881000406|bibcode=1988AngCh.100..495L |issn=1521-3757}} While these elements have been described historically as completely inert, chemical compounds have been synthesized with all but neon and helium.{{Cite journal|last=Christe|first=Karl O.|date=2001-04-17|title=A Renaissance in Noble Gas Chemistry|url=http://onlinelibrary.wiley.com/doi/10.1002/1521-3773(20010417)40:83.0.CO;2-J/abstract|journal=Angewandte Chemie International Edition|language=en|volume=40|issue=8|pages=1419–1421|doi=10.1002/1521-3773(20010417)40:8<1419::aid-anie1419>3.0.co;2-j|pmid=11317290|issn=1521-3773|url-access=subscription}}

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.

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 kinetic energy of a single atom of a monatomic gas at thermodynamic temperature T is given by $\backslash frac\{3\}\{2\}k\_\backslash text\{B\}T$, where k_B is the Boltzmann constant. One mole of atoms contains an Avogadro number ($N\_\backslash text\{A\}$) of atoms, so that the energy of one mole of atoms of a monatomic gas is $\backslash frac\{3\}\{2\}\; k\_\backslash text\{B\}\; T\; N\_\backslash text\{A\}\; =\; \backslash frac\{3\}\{2\}\; RT$, where R is the gas constant.

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:[http://physics.bu.edu/~redner/211-sp06/class-thermodynamics/heatcap_volume.html Heat Capacity of an Ideal Gas][https://ccrma.stanford.edu/~jos/pasp/Heat_Capacity_Ideal_Gases.html Heat Capacity of Ideal Gases][http://depts.washington.edu/chemcrs/bulkdisk/chem152A_aut10/notes_Lecture_03_100610_revised.pdf Lecture 3: Thermodynamics of Ideal Gases & Calorimetry]{{dead link|date=February 2018 |bot=InternetArchiveBot |fix-attempted=yes }}, p. 2

{{block indent|1=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).}}

{{block indent|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).}}

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Category:Gases