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Why is the boiling point and melting point of 15th group and 16th group has an exception?

We know that as molecular mass increases boiling point and melting point also increase. So, down the group 15 and 16, as molecular mass increases, melting and boiling point should also increase but melting and boiling point of antimony is more than that of bismuth. (even though molecular mass of bismuth is more)

Similarly, melting and boiling point of tellurium is higher than polonium. Why is it so?

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    $\begingroup$ Your "rule" holds for the non-metals in groups 18,17,16,15. In group 14,13 it's total nonsense, and also beneath the diagonal. Look at the structure of those elements. (Di-)Atomic gases and one liquid, more complex molecules, huge macromolecules, and metals. Why should they all follow this one oversimplicistic rule? $\endgroup$ – Karl Aug 31 '15 at 19:01
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    $\begingroup$ @Karl I'm retagging this question; and I believe that's a fine response in itself. Just add why that doesn't work for metals; and huzzah! $\endgroup$ – M.A.R. ಠ_ಠ Aug 31 '15 at 19:44
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    $\begingroup$ chemistry.stackexchange.com/questions/33815/… $\endgroup$ – Nilay Ghosh Dec 12 '16 at 11:49

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