Rigid molecules have only three translational degrees of freedom and two rotational degrees of freedom, and the specific heat capacity C=5R/2.

Non-rigid molecules have three translational degrees of freedom, two vibrational degrees of freedom and two rotational degrees of freedom, and the specific heat capacity is 7R/2.

According to the equipartition theorem of energy, the energy of each degree of freedom can be obtained.

Energy kT/2 of one degree of freedom. For example, in the question (1), the translational kinetic energy is 3NkT/2, and the internal energy is 5NkT/2.