`C_(v)` values of He is always `(3R)/(2)` but `C_(v)` values of `H_(2)` is `(3R)/(2)`
at low temperature and `(5R)/(2)` at higher temperature explain .

Answer

Explainging by using vibrasitional modes

Explanation

In case of helium (monoatomic gas) we have onlu three degrees of freedom which correspond to three translational motion so the tota heat capacity will increase. The contributors by vibrational motion is not appreciable at low temperature but increase from 0 to R when temperature increase.
`C_(V) = - f (R)/(2)`, where f is the degree of freedom. At low temperature only translational motion is considered and `f = 3. :. C_(V) = (3R)/(2)` At moderate temperature both translational and rotational motions are considered. `f = 3 + 2` (3-translational and 2 rotational). `:. C_(V) = (5R)/(2)`. At still high temperature translational, rotational and vibrational motions are considered. `f = 3 + 2 + 2` (3-translatinal , 2-rotational, 2-vibrational). `:. C_(V) = (7R)/(2)`