Alkaline earth metalElement 20 · s-block

Calcium (Ca)

Calcium (Ca) is an alkaline earth metal with atomic number 20 and a standard atomic weight of 40.078. At room temperature it is a solid that melts at 842 °C and boils at 1,484 °C.

20CaCalcium40.078

Atomic number

protons

Atomic weight

40.078

CIAAW 2024

Melting point

842 °C

1,115.15 K

Boiling point

1,484 °C

1,757.15 K

Density

1.54 g/cm³

near room temperature

Electronegativity

Pauling scale

State at 25 °C

Solid

Discovered

1808

England

Calcium at a glance

Identity

Symbol	Ca
Atomic number (Z)	20
Atomic weight	40.078 ± 0.004 (CIAAW 2024)
Category	Alkaline earth metal
Group	2 — alkaline earth metals
Period	4
Block	s-block
CAS number	7440-70-2

Atomic properties

Electron configuration	[Ar] 4s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
Electrons per shell	2, 8, 8, 2
Oxidation states	+2 Less common: +1
Electronegativity (Pauling)	1
First ionisation energy	6.113 eV (589.8 kJ/mol) 2nd: 11.872 eV · 3rd: 50.913 eV
Electron affinity	0.025 eV
Atomic radius (empirical)	180 pm
Covalent radius	171 pm
Van der Waals radius	231 pm

Physical properties

State at 25 °C	Solid
Melting point	842 °C · 1,547.6 °F · 1,115.15 K
Boiling point	1,484 °C · 2,703.2 °F · 1,757.15 K
Density	1.54 g/cm³
Specific heat capacity	0.647 J/(g·K)
Crystal structure	Face-centred cubic

Isotopes & abundance

Natural occurrence	Primordial
Radioactive	No
Natural isotopes (abundance)	⁴⁰Ca 96.941%, ⁴⁴Ca 2.086%, ⁴²Ca 0.647%, ⁴⁸Ca 0.187% (radioactive), ⁴³Ca 0.135%, ⁴⁶Ca 0.004%
Stable isotopes	7
Known isotopes	29
Abundance in Earth's crust	41,500 mg/kg
Abundance in seawater	412 mg/L

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Calcium electron configuration

Calcium has 20 electrons. Written in short form, its ground-state configuration is [Ar] 4s² — the electrons of the noble gas argon, plus 4s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².

As a member of group 2 it has 2 valence electrons in its outer shell (shell 4), which largely decide its chemistry.

4s↑↓

Discovery and name

Discovered

1808 — Humphry Davy · England.

Name

Latin calx, lime.

Known for

The fifth most abundant element in Earth's crust. Calcium builds bones and teeth; limestone and cement (calcium carbonate and oxides) are construction staples.

Isotopes of calcium

Naturally occurring calcium is a mix of 6 isotopes. Each has 20 protons; they differ in the number of neutrons:

Isotope	Protons	Neutrons	Natural abundance	Stability
⁴⁰Ca	20	20	96.941%	stable
⁴⁴Ca	20	24	2.086%	stable
⁴²Ca	20	22	0.647%	stable
⁴⁸Ca	20	28	0.187%	radioactive · half-life 56 × 10¹⁸ years
⁴³Ca	20	23	0.135%	stable
⁴⁶Ca	20	26	0.004%	stable

29 isotopes of calcium are known in our data set (half-lives as published in nuclear data tables).

Calcium compared with group 2

The other elements in group 2 (alkaline earth metals), from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Beryllium	4	9.0122	1,287 °C	2,468 °C	1.85 g/cm³	1.57
Magnesium	12	24.305	650 °C	1,090 °C	1.74 g/cm³	1.31
Calcium	20	40.078	842 °C	1,484 °C	1.54 g/cm³	1
Strontium	38	87.62	777 °C	1,377 °C	2.64 g/cm³	0.95
Barium	56	137.33	727 °C	1,845 °C	3.51 g/cm³	0.89
Radium	88	[226]	696 °C	1,737 °C	5 g/cm³	0.9

Frequently asked questions

6 answers

What is the atomic mass of calcium?

The standard atomic weight of calcium is 40.078 ± 0.004 (CIAAW 2024), so one mole weighs about 40.078 grams.

How many protons, neutrons and electrons does calcium have?

A neutral calcium atom has 20 protons and 20 electrons. Its most common isotope, calcium-40, has 20 neutrons.

What is the electron configuration of calcium?

[Ar] 4s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²).

What are the melting and boiling points of calcium?

Calcium melts at 842 °C (1,547.6 °F) and boils at 1,484 °C (2,703.2 °F).

Is calcium a metal?

Calcium is a metal. It is in group 2, period 4, and is a solid at room temperature.

Who discovered calcium?

1808 — Humphry Davy (England).