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How do we reconcile these two statements?

  1. Formation of electron pairs is energetically favorable. Therefore, free radicals are particularly reactive
  2. Hund's rule - it takes energy to pair electrons, so orbitals are typically filled singly first
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The first statement in the question is not correct. Formation of electron pairs is not favorable. That would lead to putting two electrons in the same orbital and there is some effect from charge-charge repulsion.

Electrons end up paired in bonding or stabilized orbitals because there is space for a second electron, and the overall stabilization that would result from putting a second electron in is greater than the destabilization of getting that electron from somewhere else.

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A free radical is generated by homolytic bond cleavage of a covalent bond and each of the elements have one electron each.This makes them very reactive.Whereas Hund's rule is based on the fact that in electron pairing each orbital should be singly filled.

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  • $\begingroup$ This doesn't provide an answer to the main question, since the OP is trying to find why the two seem contradictory per the explanation given in the question. $\endgroup$ – M.A.R. Aug 29 '16 at 6:49

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