Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues strontium and barium. It is the fifth most abundant element in Earth's crust, and the third most abundant metal, after iron and aluminium. The most common calcium compound on Earth is calcium carbonate, found in limestone and the fossilised remnants of early sea life; gypsum, anhydrite, fluorite, and apatite are also sources of calcium. The name derives from Latin calx "lime", which was obtained from heating limestone.

Some calcium compounds were known to the ancients, though their chemistry was unknown until the seventeenth century. Pure calcium was isolated in 1808 via electrolysis of its oxide by Humphry Davy, who named the element. Calcium compounds are widely used in many industries: in foods and pharmaceuticals for calcium supplementation, in the paper industry as bleaches, as components in cement and electrical insulators, and in the manufacture of soaps. On the other hand, the metal in pure form has few applications due to its high reactivity; still, in small quantities it is often used as an alloying component in steelmaking, and sometimes, as a calcium–lead alloy, in making automotive batteries.

Calcium is the most abundant metal and the fifth-most abundant element in the human body. As electrolytes, calcium ions (Ca2+) play a vital role in the physiological and biochemical processes of organisms and cells: in signal transduction pathways where they act as a second messenger; in neurotransmitter release from neurons; in contraction of all muscle cell types; as cofactors in many enzymes; and in fertilization. Calcium ions outside cells are important for maintaining the potential difference across excitable cell membranes, protein synthesis, and bone formation.

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Atomic properties

Standard atomic weight40.078 ±0.004
Atomic mass40.0784 u

Atomic radii

Radius (empirical)197 pm
Radius (calculated)194 pm
Covalent radius176 ±10 pm
Van der Waals radius231 pm

Atomic shell

Electron configurationAr 4s2
Ionization energy(1st) 6.1131554 eV
(2nd) 11.871719 eV
(3rd) 50.91316 eV
(4th) 67.2732 eV
(5th) 84.34 eV
Shell model

Physical properties

Density1.55 g·cm−3 (293.1 K)
Molar volume2.62·10-5 m3·mol−1
Speed of sound3,810 m·s−1 (293.1 K)


Melting point1,115 K
Boiling point1,757 K
Liquid range642 K
Transition temperature15 K


Melting enthalpy8.54 kJ·mol-1
Enthalpy of vaporization155 kJ·mol-1
Binding energy178 kJ·mol-1

Heat and conductivity

Specific heat capacity647.3 J·kg−1·K−1
Thermal conductivity200 W·m-1·K-1
Expansion coefficient2.23·10-5 K-1
Work function2.87 eV


Mohs hardness1.75
Brinell hardness167 NM·m-2

Elastic properties

Young’s modulus20 GPa
Shear modulus7.4 GPa
Bulk modulus17 GPa
Poisson’s ratio0.31

Electrical properties

Electrical conductivity2.94·107 S·m-1
Resistance3.4·10-8 Ωm


Magnetic susceptibility4·10-5 cm3·mol−1

Chemical properties

Basicitystrongly basic
Oxidation state(+1) +2
Standard potential-2.84 V (Ca2+ + 2e- → Ca)


Pauling scale1
Sanderson scale0.95
Allred-Rochow scale1.04
Mulliken scale1.08
Allen scale1.034
Ghosh-Gupta scale2.696 eV
Boyd-Edgecombe scale1.34
Nagle scale1.11
Pearson absolute negativity2.2 eV

Other properties

Natural occurrenceprimordial
Crystal structureFace-centered cubic
Goldschmidt Classificationlithophile
Superconductorwith transition tempperature under special conditions
Price/kg2.28 ±0.14 USD

Natural abundances

70,000 ppb ≈ 7.03·1014 M☉
70,000 ppb ≈ 1.39·1014 Mt
1.1·107 ppb ≈ 11 kg
Earth’s crust
5·107 ppb ≈ 1.38·109 Mt
4,220 ppb ≈ 5.78 Mt
Flowing water
1,500 ppb ≈ 24 Gt
Human body
1.4·107 ppb ≈ 980 g