Say, for example, I wanted to compare the standard entropies of $\ce{Cl2(g)}$ and $\ce{F2(g)}$. They both are equimolar.

I understand that entropy is a measure of how many states a particle can occupy (loosely speaking), so I tend to think the larger chlorine molecule has higher standard entropies. This is backed by experimental data, but why?

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    $\begingroup$ You can calculate the entropy exactly using the partition function from statistical mechanics for rotations, vibration and any electronic component plus the Sakur-Tetrode equation for the translational entropy $S_T$. This depends on the mass and the partition functions depends on vibrational frequency and moment of inertial. If $Z$ is the partition function then S=R(\ln(Z)+Td\ln(Z)/dT). The partition function is $Z=\sum_i g_iE^{E_i/k_BT}$ for levels with energy $E_i$ and degeneracy $g$. Phys Chem text book give formulas for calculating $Z$. The total entropy $=S_{vib}+S_{rot}+S_{elec}+S_T$ $\endgroup$ – porphyrin Apr 11 '18 at 15:01

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