Atomic number 20 · Alkaline earth metal
Calcium Ca
Calcium is a reactive alkaline earth metal and the fifth most abundant element in Earth's crust, found in limestone, marble, chalk and gypsum. It is the most abundant metal in the human body, building bones and teeth and carrying signals that make muscles contract and blood clot.
Key properties
| Atomic number | 20 |
|---|---|
| Atomic weight | 40.078 (CIAAW 2024 abridged standard atomic weight) |
| Category | Alkaline earth metal |
| Group | 2 |
| Period | 4 |
| Block | s |
| State (25 °C, 1 atm) | Solid |
| Melting point | 1115 K (841.9 °C) |
| Boiling point | 1757 K (1483.8 °C) |
| Density | 1.54 g/cm³ |
| Electronegativity (Pauling) | 1 |
| Atomic radius (van der Waals) | 231 pm |
| First ionization energy | 6.113 eV |
| Electron affinity | — |
| Oxidation states | +2 |
| Discovered | 1808 |
Electron configuration
- Condensed
- [Ar]4s2
- Full
- 1s2 2s2 2p6 3s2 3p6 4s2
- Electrons per shell
- 2 · 8 · 8 · 2
Orbital diagram
Position in the table
Discovery
Calcium compounds have been used since prehistoric times. Burning limestone produces lime, which ancient Egyptians, Greeks and Romans used in plaster and mortar; Roman concrete relied on lime mixed with volcanic ash. Gypsum plaster was used in Egyptian buildings, and chalk has long served for drawing and writing.
Lime could not be broken down by heat or chemicals, and Lavoisier listed it among the 'earths'. In 1808 Humphry Davy, building on a method suggested by Jöns Jacob Berzelius and Magnus Pontin, electrolysed a mixture of lime and mercuric oxide to form a calcium amalgam, then distilled away the mercury to leave a small amount of calcium metal. Pure calcium in larger quantities was not produced until the early twentieth century.
Origin of the name
Davy derived the name from the Latin calx (genitive calcis), meaning lime or limestone. The same root appears in calculate, from the small stones once used for counting. The symbol is Ca.
Main uses
- Construction: limestone, lime and cement, all calcium compounds, are among the most heavily used materials on Earth.
- Steelmaking: lime removes impurities such as phosphorus and sulfur, and calcium metal is added to deoxidise and refine steel.
- Metal production: calcium metal reduces the oxides or fluorides of metals such as uranium and thorium.
- Fillers and plaster: calcium carbonate fills paper, plastics and toothpaste, and gypsum makes plasterboard.
- Batteries and alloys: small amounts of calcium strengthen the lead grids in some lead-acid batteries.
Isotopes
Calcium has six naturally occurring isotopes. Calcium-40 makes up about 97 percent; calcium-42, calcium-43, calcium-44 and calcium-46 are stable, and calcium-48 is so long-lived that it behaves as stable. Calcium-48 has an unusually neutron-rich, especially stable nucleus, and beams of it were used to create the superheavy elements 114 to 118. The radioactive calcium-41 (half-life about 100,000 years) is studied for dating bones, and calcium-45 (about 163 days) is used as a tracer in biology.
In everyday life
An adult body contains roughly a kilogram of calcium, about 99 percent of it in bones and teeth. The rest works as a messenger inside cells, triggering muscle contraction, nerve signalling and blood clotting. Dairy products, tofu, small fish eaten with bones and leafy greens are good sources, and vitamin D helps the body absorb it.
Hard water is rich in calcium and leaves limescale in kettles, and the most common type of kidney stone is calcium oxalate.
Good to know
- Calcium metal melts at about 1,115 K (842 °C) and slowly reacts with water to release hydrogen.
- Calcium salts give a flame a brick-red to orange colour.
- The white cliffs of Dover are made of chalk, the calcium carbonate skeletons of tiny marine algae.
Same group (2)
Data sources · Properties: PubChem Periodic Table (US NIH/NLM public data) · Atomic weights: CIAAW 2024 abridged standard atomic weights · Names: Wikidata (CC0); Korean names follow the Korean Chemical Society. Retrieved 2026-09-23.