Calcium (Ca)
Element 20 · Alkaline earth metal · atomic mass 40.078 u · group 2, period 4
Electron shells: 2, 8, 8, 2
Calcium is the fifth most abundant element in the crust and the structural mineral of bone, shell and coral. As lime and cement it also literally holds up cities.
The white cliffs of Dover are the compressed calcium skeletons of trillions of plankton.
- Atomic number
- 20
- Atomic mass
- 40.078 u
- Electron configuration
- [Ar] 4s²
- Group · period · block
- 2 · 4 · s
- Oxidation states
- +2
- Electronegativity
- 1 (Pauling)
- Melting point
- 842 °C
- Boiling point
- 1,484 °C
- Density
- 1.54 g/cm³
- State at 20 °C
- Solid
- Appearance
- Dull silvery-grey metal
- Discovered
- Humphry Davy, 1808
20 protons, and 20 electrons when neutral
IUPAC standard atomic weight
Alkaline earth metals
What Calcium is used for
- Cement and concrete
- Steelmaking flux
- Dietary supplements
- Plaster and mortar
Atomic structure of Calcium
A neutral calcium atom has 20 protons and 20 electrons. Its most common isotope carries about 20 neutrons, which is where the mass number 40 comes from. Change the neutron count and you have a different isotope of calcium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 8, 2. In orbital notation that is [Ar] 4s², leaving 2 valence electrons in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Calcium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Calcium sits in the periodic table
Group 2 metals with two outer electrons. Harder and less reactive than the alkali metals, and they form the +2 ions that build bone, shell and cement.
Discovery and naming
Calcium was discovered by Humphry Davy in 1808. Its name comes from Latin calx, "lime".
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Calcium?
The standard atomic weight of Calcium is 40.078 u, as published by IUPAC's Commission on Isotopic Abundances and Atomic Weights.
How many protons, neutrons and electrons does Calcium have?
Calcium has 20 protons, because the atomic number is the proton count and that is what makes an atom Calcium at all. A neutral Calcium atom has the same number of electrons, 20. The neutron count varies by isotope; the most common one has about 20, since 40 − 20 = 20.
What is the electron configuration of Calcium?
The electron configuration of Calcium is [Ar] 4s². Its electrons are arranged 2, 8, 8, 2 across 4 shells.
Is Calcium a metal?
Yes. Calcium is classified as a alkaline earth metal, on the left side of the table.
What group and period is Calcium in?
Calcium is in group 2 (the alkaline earth metals), period 4, in the s-block. The group is its column and the period is its row.
What is the charge of Calcium?
Calcium most commonly forms a 2+ ion. It has 2 electrons in its outer shell and loses them readily, which is what the group number tells you.
What is the melting point of Calcium?
Calcium melts at 842 °C and boils at 1,484 °C. That puts it in the solids at everyday temperatures.
What is Calcium used for?
Cement and concrete, Steelmaking flux, Dietary supplements, Plaster and mortar. Calcium is the fifth most abundant element in the crust and the structural mineral of bone, shell and coral.
Who discovered Calcium, and where does its name come from?
Calcium (Ca) was discovered by Humphry Davy in 1808. The name comes from Latin calx, "lime".