The value of for hydrogen, helium and another ideal diatomic gas X (whose molecules are not rigid but have an additional vibrational mode), are respectively equal to,
Diatomic gases have 5 degrees of freedom, if vibrational mode is neglected. And if vibrational mode is also considered then degrees of freedom of diatomic gas molecules are 7.
For Hydrogen
For Helium
For gas X (vibrational mode also considered) f = 7.