Number of degrees of freedom of a **rigid diatomic molecule** (e.g. O₂ at room temperature, vibrations not excited):
A3
B5 (3 translational + 2 rotational)
C6
D7
Answer & Solution
Correct answer: B. 5 (3 translational + 2 rotational)
Rigid diatomic: 3 translational + 2 rotational (about two axes perpendicular to molecular axis; rotation about the axis itself contributes negligibly). $C_V = (5/2)R$, $C_P = (7/2)R$, $\gamma = 7/5 = 1.4$.
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