Editing Period 4 element (section)

===Atomic structure===
Progressing towards increase of [[atomic number]], the [[Aufbau principle]] causes elements of the period to put [[electron]]s onto 4s, 3d, and 4p subshells, in that order. However, there are exceptions, such as [[#Chromium|chromium]]. The first twelve elements—[[#Potassium|K]], [[#Calcium|Ca]], [[#d-block elements|and transition metals]]—have from 1 to 12 [[valence electron]]s respectively, which are placed on 4s and 3d.

Twelve electrons over the [[electron configuration]] of [[argon]] reach the configuration of [[#Zinc|zinc]], namely 3d<sup>10</sup> 4s<sup>2</sup>. After this element, the filled 3d subshell effectively withdraws from chemistry and the subsequent [[periodic trends|trend]] looks much like trends in the periods [[period 2 element|2]] and [[period 3 element|3]]. [[#p-block elements|The p-block elements]] of period&nbsp;4 have their [[valence shell]] composed of 4s and 4p subshells of the [[electron shell|fourth ({{math|1=''n'' = 4}}) shell]] and obey the [[octet rule]].

For [[quantum chemistry]] namely this period sees transition from the simplified [[electron shell]] paradigm to research of many [[azimuthal quantum number|differently-shaped subshells]]. The relative disposition of their [[energy level]]s is governed by the interplay of various physical effects. The period's [[#s-block elements|s-block metals]] put their differentiating electrons onto 4s despite having vacancies among nominally lower [[principal quantum number|{{math|1=''n'' = 3}} states]]&nbsp;– a phenomenon unseen in lighter elements. Contrariwise, the six elements from [[#Gallium|gallium]] to [[#Krypton|krypton]] are the heaviest where all electron shells below the valence shell are filled ''completely''. This is no longer possible in further periods due to the existence of f-subshells starting from {{math|1=''n'' = 4}}.