Heat capacity ratio of argon

A monatomic gas stores energy only in translation — three degrees of freedom, no rotation or vibration to speak of — so c_v = (3/2)R, c_p = (5/2)R and γ = 5/3 = 1.6667. Argon, helium, neon, krypton and xenon all measure within a fraction of a percent of it over enormous temperature ranges, which is about as clean as physics gets.

That reliability is why argon is the working gas of choice for acoustic thermometry, the technique that measured the Boltzmann constant precisely enough to redefine the kelvin in 2019: with γ known exactly from theory, the speed of sound in argon gives kT/m directly. The high γ is also why helium and argon heat up so much in adiabatic compression, and why helium-oxygen mixes change a diver's voice.