If vibrations are excited, a diatomic molecule has DOF:
A5
B6
C7 (3 trans + 2 rot + 2 vib (KE + PE))
D9
Answer & Solution
Correct answer: C. 7 (3 trans + 2 rot + 2 vib (KE + PE))
At high T, vibrational modes 'unlock' — diatomic vibration contributes 2 DOF (KE + PE both quadratic). Total = 3 + 2 + 2 = 7. $C_V = (7/2)R$. Realised only at high temperatures.
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