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I stumbled upon an exercise asking to draw the resonance structure of $\ce{SO4^{2-}}$.

We can choose between the resonance structures by calculating the formal charges and by checking which one gives out the least value, and which one has the negative charges on the more electronegative atom.

My Chemistry Textbook "Chemistry 10th edition" by Chang states that:

The general rule for elements in the third period and beyond is that a resonance structure that obeys the octet rule is preferred over one that involves an expanded octet but bears fewer formal charges.

As I've searched more about this example , I've found out that most exercises that treated this molecule have labeled the one that has the least formal charge as the more stable.

This got me confused, is the resonance structure which obeys the octet rule the more plausible or is it the one which has the least formal charge?

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    $\begingroup$ Your textbook is correct. $\endgroup$ – Mithoron Feb 2 '18 at 22:07
  • $\begingroup$ @Mithoron is right. The "expanded octet" model was built to fit molecules like $\ce{SF_6}$ to pairwise bonds. Today we use molecular orbitals instead of pairwise bonds, and the molecular orbitals can be (and are) constructed without expanded octets -- including the case of $\ce{SF_6}$. $\endgroup$ – Oscar Lanzi Feb 2 '18 at 22:14
  • $\begingroup$ chemistry.stackexchange.com/questions/47086/… $\endgroup$ – Mithoron Feb 2 '18 at 22:17