The molar heat capacity , `C_(v)` of helium gas is `3//2R` and is independent of temperature. For hydrogen gas, `C_(v)` approaches `3//2R` at very low temperature, equal `5//2R` at moderate temperature and is higher than `5//2 R` at high temperatures. Give a reason for the temperature dependence of `C_(v)` in case of hydrogen, in not more than two or three sentences.

Explanation

He is monoatomic, so it has only three degree of freedom (translational only) at all temperature hence always `3/2 R`.
Hydrogen molecule is diatomic, has three translational, two rotational and one vibrational degree of freedom. The energy spacing between adjacent levels are in the order of
Translational `lt` rotational `lt` vibrational
At lower temperature only translational degree of freedom contribute to heat capacity while at higher temperature rotational and vibrational degree of freedom starts contributing to heat capacity.