Question

How can atoms exceed the octet rule?

Answer 1

Good question!

The octet rule is based on the availability of an s and three p orbitals holding eight electrons. For the n=2 shell (second row of the periodic table) any additional electrons requires opening up another shell--energetically expensive.

For the n=3 shell the 3s and 3p orbitals are available to accommodate an octet. However, for n=3 there are also the 3d orbitals available, albeit at higher energy than the s and p orbitals. So any element from the third or later periods technically has the ability to break the octet rule.

For n=2 (C, N, O, F) the octet rule is iron-clad. The third and later period non-metals can break the rule. For example, the sulfate anion can be drawn as a tetrahedral of singly-bonded oxygens, but the more correct way to draw it is two double-bond oxygens and two single-bond oxygens. Similarly for sulfur hexafluoride (`SF_6`). Sulfur has clearly broken the octet rule with six bonds and twelve electrons.

Cheers!