By Nick B. - Own work, [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0"Creative Commons Attribution-Share Alike 4.0"), [Link](https://commons.wikimedia.org/w/index.php?curid=54031939)
By Nick B. - Own work, [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0 "Creative Commons Attribution-Share Alike 4.0"), [Link](https://commons.wikimedia.org/w/index.php?curid=54031939)
To explain assume that each atom is a classical harmonic oscillator in 3D.
This gives 6 degrees of freedom, each contributing $k_B/2$ to the heat capacity.