Editing Period 4 element (section)

==Properties==

All 4th-period elements are [[Radioactive element|stable]],<ref>{{cite web|url=https://www.science.co.il/elements/?s=Earth |title=List of Elements of the Periodic Table – Sorted by Abundance in Earth's crust |publisher=Science.co.il |access-date=2012-08-14}}</ref> and many are extremely common in the [[Earth's crust]] and/or [[Earth's core|core]]; it is the last period with no unstable elements. Many transition metals in the period are very [[Strength of materials|strong]], and therefore common in [[industry (manufacturing)|industry]], especially [[#Iron|iron]].<ref name=Gray /> Some are [[toxic]], with all known [[#Vanadium|vanadium]] compounds toxic,<ref>{{cite journal |last=Srivastava |first=A. K. |year=2000 |title=Anti-diabetic and toxic effects of vanadium compounds |journal=Molecular and Cellular Biochemistry |volume=206 |issue=206 |pages=177–182 |doi=10.1023/A:1007075204494 |pmid=10839208 |s2cid=8871862}}</ref> [[#Arsenic|arsenic]] one of the most well-known [[poison]]s, and [[#Bromine|bromine]] a toxic liquid. Conversely, many elements are essential to human survival, such as [[#Calcium|calcium]], the main component in [[bones]].<ref name="Gray" >{{cite book |last=Gray |first=Theodore |title=The Elements: A Visual Exploration of Every Known Atom in the Universe |year=2009 |publisher=Black Dog & Leventhal Publishers |location=New York |isbn=978-1-57912-814-2 |url-access=registration |url=https://archive.org/details/elementsvisualex0000gray }}</ref>

===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}}.