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In chemistry, it is agreed in general that the octet is extremely stable, through experiments except in some cases. But is there any specific theoretical reasoning for that?

According to me, if:

  1. The $\ce{s}$ and $\ce{p}$ orbitals are full, they should repel in every orbital, causing destabilization.
  2. There is no exchange of energy in fully-filled orbitals as it occurs in half filled orbitals.

So, shouldn't these two reasons make an octet filled atom unstable? Then, why is the octet stable and how does it counter the above arguments?

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  • $\begingroup$ Consider adding an electron and removing an electron... $\endgroup$ – Zhe May 8 '18 at 13:06
  • $\begingroup$ It isn't always so. Try drawing a structure for a cyclopropenyl C3H3 ring with one double bond, putting the right charge on it to make every carbon atom have an octet. But, the actually stable species has two electrons less than what you drew! The link given above tells why. $\endgroup$ – Oscar Lanzi May 8 '18 at 13:08