The number of degrees of freedom ($f$) is the number of independent ways a molecule can possess energy.

• Translational: All gas molecules have 3 translational degrees of freedom ($x,y,z$ directions).
• Rotational: A linear diatomic molecule (like $H_2,O_2,N_2$) can rotate about two axes perpendicular to the bond axis. Rotation about the bond axis itself has negligible moment of inertia and doesn't contribute at moderate temperatures.
• Total $f$ = 3 (trans) + 2 (rot) = 5. Note: At very high temperatures, 2 vibrational degrees of freedom may also activate.