Calcium
Element 20 · Alkaline Earth Metal
Calcium is a soft, gray alkaline earth metal in group 2. It reacts with water more gently than the alkali metals, producing hydrogen gas and calcium hydroxide, and it burns in air with a brilliant orange-red flame. It always forms the +2 ion in its compounds.
Calcium's greatest role is structural: it hardens bones and teeth in animals, and its carbonate and silicate compounds are the literal bedrock of construction — limestone, marble, chalk, cement, and concrete. Industrially, calcium oxide (quicklime) and calcium hydroxide (slaked lime) are among the most produced chemicals in the world.
Key facts
| Atomic number | 20 |
|---|---|
| Atomic weight | 40.078 |
| Category | Alkaline Earth Metal |
| Group / Period | 2 / 4 |
| Block | s |
| Electron configuration | [Ar] 4s2 |
| Common oxidation states | +2 |
| State at STP | Solid |
| Electronegativity | 1.00 |
| Atomic radius | 194 pm |
| Density | 1.55 g/cm³ |
| Melting point | 842 °C |
| Boiling point | 1484 °C |
Occurrence
Calcium is the fifth most abundant element in Earth's crust but never occurs as a free metal. It is locked in minerals such as limestone and chalk (calcium carbonate), gypsum (calcium sulfate), and fluorite (calcium fluoride). Oceans hold large dissolved reserves, and corals, shells, and skeletons are built from its compounds.
Applications
Cement and concrete rely on calcium compounds as the binding ingredient
Steelmaking uses calcium to remove sulfur and oxygen impurities
Plaster of Paris and drywall are made from calcium sulfate minerals
Antacid tablets neutralize stomach acid with calcium carbonate
Calcium metal alloys strengthen aluminium and lead products
Common compounds
- calcium carbonate
- calcium oxide
- calcium hydroxide
- calcium sulfate
- calcium fluoride
- calcium chloride
- calcium phosphate
About 99% of the calcium in your body is stored in your bones and teeth.
Data source: IUPAC standard atomic weights 2021