Editing Period 4 element (section)

===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}}