Editing Octet rule (section)

== Explanation in quantum theory ==

The quantum theory of the atom explains the eight electrons as a [[closed shell]] with an s<sup>2</sup>p<sup>6</sup> electron configuration. A closed-shell configuration is one in which low-lying energy levels are full and higher energy levels are empty. For example, the [[neon]] atom ground state has a full {{nowrap|1=''n'' = 2}} shell (2s<sup>2</sup>2p<sup>6</sup>) and an empty {{nowrap|1=''n'' = 3}} shell. According to the octet rule, the atoms immediately before and after neon in the periodic table (i.e. C, N, O, F, Na, Mg and Al), tend to attain a similar configuration by gaining, losing, or sharing electrons.

The [[argon]] atom has an analogous 3s<sup>2</sup>3p<sup>6</sup> configuration. There is also an empty 3d level, but it is at considerably higher energy than 3s and 3p (unlike in the hydrogen atom), so that 3s<sup>2</sup>3p<sup>6</sup> is still considered a closed shell for chemical purposes. The atoms immediately before and after argon tend to attain this configuration in compounds. There are, however, some [[hypervalent molecule]]s in which the 3d level may play a part in the bonding, although this is controversial (see below).

For [[helium]] there is no 1p level according to the quantum theory, so that 1s<sup>2</sup> is a closed shell with no p electrons. The atoms before and after helium (H and Li) follow a duet rule and tend to have the same 1s<sup>2</sup> configuration as helium.