← Periodic table · Alkaline earth metals

Calcium (Ca)

Calcium (symbol Ca) is the chemical element with atomic number 20: an alkaline earth metal, solid at 0 °C, with an atomic mass of 40.078 u. Discovered in 1808 (Humphry Davy).

Calcium is a silvery metal we never see in that form, because it reacts with water and air. We know it through its compounds: limestone, chalk, marble, plaster and cement. In living things it builds bones, teeth and shells, and acts as a messenger for muscle contraction. It is the most abundant mineral in the human body.

Key facts

Atomic number
20
Symbol
Ca
French name
Calcium
Atomic mass
40.078 u
Molar mass
40.078 g/mol
Family
Alkaline earth metals
Position
period 4, group 2, s-block
State at 0 °C
Solid
Melting point
841.9 °C
Boiling point
1,483.9 °C
Density
1.55 g/cm³
Electronegativity (Pauling)
1
First ionization energy
590 kJ/mol
Atomic radius
180 pm
Electron configuration
[Ar] 4s²
Abundance in Earth's crust
41,500 ppm (5th most abundant element)
Indicative price
2 $/kg (order of magnitude)
Radioactive
no
Discovery
1808, Humphry Davy

Origin of the name

From the Latin calx, lime.

What does calcium look like?

A fairly soft, silvery-gray metal that quickly develops a white coating of oxide and hydroxide in air.

Where is calcium found?

About 4.1% of Earth's crust, the fifth most abundant element. Vast limestone massifs are made of the skeletons and shells of marine organisms that piled up on the seafloor. The human body contains about 1 kg, 99% of it in bones and teeth.

Natural sources

  • Calcite (limestone, chalk, marble)
  • Aragonite
  • Gypsum
  • Dolomite
  • Fluorite
  • Apatite

How is calcium produced?

Limestone is quarried in colossal amounts: it goes into about 4 billion tonnes of cement and more than 400 million tonnes of lime each year. The metal, made in small quantities, is obtained by reducing lime with aluminium under vacuum.

Top producers

  • China
  • India
  • United States
  • Russia

What is calcium used for?

  • Cement, concrete and mortar.
  • Lime for steelmaking, water treatment and agriculture.
  • Plaster (gypsum) for construction.
  • Dietary supplements and antacids.
  • The metal is used to extract other metals (uranium, thorium).

In everyday life

  • The concrete of buildings
  • The limescale furring up your kettle
  • Blackboard chalk
  • Drywall
  • Milk and cheese

Role in living things

Essential: as hydroxyapatite, it gives bones and teeth their hardness. Calcium ions trigger muscle contraction, the release of neurotransmitters and blood clotting. Corals, mollusks and coccolithophores build their skeletons from calcium carbonate.

Hazards of calcium

The metal reacts with water, giving off hydrogen. Quicklime is highly caustic and can burn skin and eyes. Too much calcium in the urine promotes certain kidney stones.

Cosmic origin

  • Supernovae: Forged in the cores of massive stars, then scattered when they explode.
  • White dwarfs: Produced by the thermonuclear explosion of a white dwarf (type Ia supernova).

History of calcium

Lime has been used for mortar for at least 9,000 years, and the Romans mastered a remarkably durable concrete. In 1808, Humphry Davy isolated the metal by electrolysis of a mixture of lime and mercury oxide. The name comes from the Latin calx, “lime.”

Did you know?

  • The white cliffs of Dover and of Étretat in Normandy are made of chalk, formed from billions of skeletons of microscopic algae (coccoliths).
  • “Plaster of Paris” owes its name to the gypsum quarried in abundance beneath Montmartre and the surrounding area.
  • Pure calcium isn't white: it is a shiny metal, and the white color comes from its compounds.

Isotopes of calcium

Six natural isotopes, a record for such a light element. Neutron-rich calcium-48 is fired as a projectile to create new superheavy elements.

  • calcium-40: 96.941% of natural calcium, stable. A “doubly magic” nucleus, exceptionally stable.
  • calcium-41: radioactive, half-life 99,400 years. Measured in trace amounts, it tracks bone loss and dates ancient bones.
  • calcium-42: 0.647% of natural calcium, stable
  • calcium-43: 0.135% of natural calcium, stable
  • calcium-44: 2.086% of natural calcium, stable. Stable tracer for studying calcium absorption and osteoporosis.
  • calcium-45: radioactive, half-life 163 days. Calcium tracer in biology and agronomy.
  • calcium-46: 0.004% of natural calcium, stable
  • calcium-47: radioactive, half-life 4.54 days. Used to study bone metabolism and to produce medical scandium-47.
  • calcium-48: 0.187% of natural calcium, stable. The beam that created elements 114 to 118; it does actually decay, but over ~6 × 10¹⁹ years.

Common compounds

Hydroxyapatite (Ca₅(PO₄)₃(OH)) · Limestone (CaCO₃) · Gypsum (plaster) (CaSO₄·2H₂O) · Fluorite (CaF₂)

Related elements

Same family (alkaline earth metals): Beryllium (Be), Magnesium (Mg), Strontium (Sr), Barium (Ba), Radium (Ra).

Same column (group 2): Beryllium (Be), Magnesium (Mg), Strontium (Sr), Barium (Ba), Radium (Ra).