What is Calcium?
{{infobox calcium}} [[File:Calcium unter Argon Schutzgasatmosphäre.jpg|thumb|Very pure calcium metal, not corroded]] [[File:Calcium.jpg|thumb|Calcium metal in air, corroded]] '''Calcium''' is a [[chemical element]]. Its symbol on the [[periodic table]] (a list of all the elements) is '''Ca'''. Its [[atomic number]] is 20. (The atomic number says where Calcium sits in the periodic table.) It has 20 [[proton]]s and 20 [[electron]]s (if is an atom, see [[ion]]). The most common [[isotope]]s are Ca-40 and Ca-44. Its [[mass number]] is about 40.08. Calcium is very important in the human body, for making [[bone]]s and for other purposes. == Properties == === Physical properties === Calcium is a soft white-gray [[metal]]. It is a solid and is opaque. It is an [[alkaline earth metal]]. Its [[melting point]] is hotter than most other reactive metals. It is a little harder than [[lead]]. It has two [[allotrope]]s. It does not conduct electricity as well as copper, but is much lighter in weight. ===Chemical properties=== [[File:Calcium air 2.theora.ogv|thumb|Calcium metal burning]] It reacts with water to produce [[hydrogen]] and [[calcium hydroxide]]. It reacts with water very fast when it is powdered. When it is in a chunk, it starts reacting slowly because calcium hydroxide makes a coating that does not dissolve on the calcium. If a little [[acid]] is added to calcium hydroxide, it dissolves it, making the calcium react very fast. It burns when powdered to make a reddish flame. This makes [[calcium oxide]]. It also makes [[calcium nitride]] when heated. It can react with [[halogen]]s to make calcium halides like [[calcium chloride]] with [[chlorine]]. ===Calcium compounds=== Calcium forms chemical compounds in the +2 [[oxidation state]]. Calcium compounds are colorless. Most calcium compounds are not toxic. They are needed in the human body, actually. They are unreactive as far as calcium ions go. Calcium oxide was used to make [[limelight]]s, which have a flame heating calcium oxide and makes it glow very bright.