Editing Period 4 element (section)

==s-block elements==
===Potassium===
[[File:Potassium-2.jpg|frameless|left|upright=0.35]]
[[Potassium]] (K) is an [[alkali metal]], underneath [[sodium]] and above [[rubidium]],<ref>{{cite web|url=http://www.tutorvista.com/content/science/science-ii/periodic-classification-elements/position-elements.php |title=Elements in the Modern Periodic Table, Periodic Classification of Elements |publisher=Tutorvista.com |access-date=2012-08-14 |archive-url=https://web.archive.org/web/20180126054026/http://www.tutorvista.com/content/science/science-ii/periodic-classification-elements/position-elements.php |archive-date=26 January 2018}}</ref> and the first element of period 4.  One of the most [[reactivity (chemistry)|reactive]] chemical elements, it is usually found only in [[Chemical compound|compound]]s.  It is a silvery metal<ref name=ChemExplained /> that tarnishes rapidly when exposed to the [[oxygen]] in air, which [[redox|oxidizes]] it. It is [[softness|soft]] enough to be cut with a knife<ref>{{cite web|url=http://education.jlab.org/itselemental/ele019.html |title=The Element Potassium |website=It's Elemental |author=Office of Science Education |publisher=[[Thomas Jefferson National Accelerator|Jefferson Science Associates]] |location=[[Newport News]] |access-date=2012-08-14}}</ref> and the second least-[[density|dense]] element.{{citation needed|date=September 2023}} [[Potassium]] has a relatively low [[melting point]]; it will melt under a small open flame.<ref name=ChemExplained>{{cite web|url=http://www.chemistryexplained.com/elements/L-P/Potassium.html |title=Potassium |website=Chemistry Explained: Foundations and Applications |publisher=[[Advameg]] |location=[[Hinsdale, IL]] |access-date=2012-08-14 |year=2023}}</ref> It also is less dense than water, and can, in principle, float<ref>{{cite web|url=http://www.lenntech.com/periodic/elements/k.htm |title=Potassium - K  |publisher=Lenntech |access-date=2012-08-14 |year=1998}}</ref> (although it will react with any water it is exposed to).<ref name=ChemExplained />

===Calcium===
[[File:Calcium unter Argon Schutzgasatmosphäre.jpg|frameless|left|upright=0.45]]
[[Calcium]] (Ca) is the second element in the period. An [[alkali earth metal]], [[native element|native]] calcium is almost never found in nature,{{citation needed|date=September 2023}} because it reacts with water.<ref>{{cite web|url=http://www.chemguide.co.uk/inorganic/group2/reacth2o.html |title=Reactions of the Group 2 elements with water |last1=Clark |first1=Jim |date=December 2021 |access-date=2012-08-14}}</ref> It has one of the most widely-known biological roles in all animals and some plants, making up structural elements such as bones and teeth.<ref>{{cite report |title=Human Vitamin and Mineral Requirements |publisher=FAO Information Division Publishing and Multimedia Service |location=Rome, Italy |year=2002 |chapter=Calcium |chapter-url=http://www.fao.org/DOCREP/004/Y2809E/y2809e0h.htm |access-date=2012-08-14}}</ref>  It also has applications in [[Cell (biology)|cells]], such as signals for [[cellular processes]]es. It is regarded as the most abundant [[mineral]] in the human body.{{citation needed|date=September 2023}}