8.31 Monatomic Gas

[Jazmin_Morales_3J]

In the question, it mentions that Kr is a monatomic gas and I was wondering what exactly is a monatomic gas? I checked in the textbook and it mentions that the molar internal energy of a monatomic ideal gas is 3/2RT. Basically, I was wondering why the internal energy formula is different for a monatomic gas?

[BrianaBarr2A]

So for an ideal gas your focusing on only one atom. But for a monatomic gas you're focusing on multiple atoms(N) so each of the three directions (x, y, and z) contribute 1/2 kT per molecule, for a total of 3/2NkT per molecule.
Hope thats what you were looking for!