Fig. 4.23 A diatomic molecule and its rotational and vibrational degree of freedom

The molecule has thus two rotational degrees of freedom. Since the atoms can also vibrate along the line joining them, the molecule has two vibrational degrees of freedom, one for kinetic energy and the other for potential energy, because the vibrational energy is partly kinetic and partly potential. A diatomic molecule is thus expected to have seven degrees of freedom (3 for translation, 2 for rotation and 2 for vibration). Thus,

These values do not tally with experimental results ( ref: Table 4.5).

Table 4.5 Experimentally measured values of C_p and γ of different gases at room temperature