When the gas consists of molecules, we need to take into account the effect of
rotation and vibration. A linear molecule, such as N2 and CO2, can rotate around two
axes perpendicular to the line of the atoms (Fig. 2.5), so it has two rotational modes
of motion, each contributing a term kT to the internal energy. Therefore, the mean
rotational energy is kT and the rotational contribution to the molar internal energy is
RT. By adding the translational and rotational contributions, we obtain