Editing Period 3 element (section)

== Atomic structure ==
In a [[quantum mechanics|quantum mechanical]] description of [[atomic structure]], this period corresponds to the buildup of electrons in the [[electron shell|third ({{math|1=''n'' = 3}}) shell]], more specifically filling its 3s and 3p subshells. There is a 3d subshell, but—in compliance with the [[Aufbau principle]]—it is not filled until [[period 4 element|period 4]]. This makes all eight elements analogs of the [[period&nbsp;2 element]]s in the same exact sequence. The [[octet rule]] generally applies to period 3 in the same way as to period 2 elements, because the 3d subshell is normally non-acting.