# Vrms for monotomic, diatomic, and polyatomic molecules

in my notes from class I have that

Vrms= sqrt(3kT/m) for a pt molecule Vrms= sqrt(5kT/m) for a diatomic molecule and that Vrms= sqrt(6kT/m) for a triatomic-> higher order molecule

says that Vrms=sqrt(3kT/m) always and I don't understand why that is.

Thanks,

I know another post has about the same question- but wasn't able to comment or add a related question due to my 0 reputation.

Don't understand where Vrms formula comes from. Is it solely dependent on translation motion. And that's why for all molecules you can use the same exact Vrms formula.

RMS Speed of Gas Molecule for Polyatomic Molecules

• It might help to explain what else confuses you? This seems to be an exact duplicate of the question you linked, and the answer there seems correct. If you could explain what you don't understand about it, then you can narrow your question down to just that aspect. – jacob1729 Jan 16 '19 at 23:47

$$\frac 12 mv^2_{\rm rms}$$ is the mean translational kinetic energy of an atom/molecule and it is the the mean translational kinetic energy which is proportional to the temperature.
The first equation is correct for all atoms/molecules but the statements about diatomic and triatomic molecules are not correct as $$\frac 52kT$$ and $$\frac 62kT$$ are equal to the total kinetic energy of a molecule not just the translational part.