Question

Which molecular motion does not contribute towards internal energy?

Answer 1

Answer:

Translational energy

Explanation:

U=f/2N k T  

f = no. of degrees of freedom

In case of an ideal monoatomic gas, the internal energy is contributed just because of the translational energy .

For instance, in the case of He or Ne, the atoms aren't bound to one another and thus they don't vibrate. Rotational energy will be ignored since their atomic moment of inertia is insignificant and higher energy excitation electronically and isn't possible(with the exception of at high temperatures).  

In this manner, the inner energy changes in an ideal monoatomic gas aren't absolutely translational and there are 3 degrees of freedom. As it can occur in 3D space, for example, x, y & z directions.  

Answer 2

internal energy.................................