Calcium
Calcium is an alkaline-earth metal that occurs naturally in minerals and as ions in living organisms.
Calcium turns chemistry into the framework of bones and landscapes.
Explore Calcium's atom- Atomic number
- 20
- Standard atomic weight
- 40.078(4)
- Family
- Alkaline earth
- Period
- 4
- Group
- 2
- Standard state
- Solid
In everyday life
Bones, teeth, limestone and chalk contain calcium compounds.
Something to remember
Limestone is largely calcium carbonate, the same compound found in many shells.
Meet its electrons
[Ar] 4s2
Published neutral-atom ground-state configuration; excited atoms and ions can differ.
Electrons per shell: 2 · 8 · 8 · 2.
The playground shows an illustrative shell model. Real electrons are described by quantum orbitals, rather than tiny planets traveling along fixed paths.
A connection to explore
Calcium and carbon are both present in calcium carbonate, a major component of limestone.
Meet Carbon (C)Properties and conditions
- Standard atomic weight
- 40.078(4)CIAAW 2024 standard atomic weight (dimensionless). Parentheses give uncertainty in the last quoted digits. Source flags: g.CIAAW standard atomic weights ↗ · Retrieved 2026-10-15
- Standard state
- SolidPubChem classification, including explicit predictions where stated. Reference temperature and pressure are not supplied in the table.PubChem periodic table ↗ · Retrieved 2026-10-15
- First ionisation energy
- 6.113 eVEnergy to remove the first electron from an isolated neutral gas-phase atom. Uncertainty is not supplied.PubChem periodic table ↗ · Retrieved 2026-10-15
- Electronegativity
- 1 Pauling scaleA dimensionless relative measure; values depend on the chosen scale. Uncertainty is not supplied.PubChem periodic table ↗ · Retrieved 2026-10-15
- Melting point
- 1115 KPubChem reported value. Pressure, allotrope and measurement uncertainty are not specified in the table; consult the cited source.PubChem periodic table ↗ · Retrieved 2026-10-15
- Boiling point
- 1757 KPubChem reported value. Pressure and measurement uncertainty are not specified in the table; consult the cited source.PubChem periodic table ↗ · Retrieved 2026-10-15
- Density
- 1.54 g/cm³PubChem reported value. Temperature, pressure, allotrope and measurement uncertainty are not specified in the table; gas densities are especially condition-dependent.PubChem periodic table ↗ · Retrieved 2026-10-15
- Oxidation states
- +2Selected PubChem oxidation states; formal electron bookkeeping, not a list of stable free-ion charges or every possible compound.PubChem periodic table ↗ · Retrieved 2026-10-15
- Discovery
- AncientPubChem discovery attribution. Ancient knowledge and competing priority claims cannot always be represented by a single year.PubChem periodic table ↗ · Retrieved 2026-10-15
- Atomic radius
- 231 pmPubChem tabulated radius. Radius definitions depend on bonding and convention; measurement definition and uncertainty are not supplied in this table.PubChem periodic table ↗ · Retrieved 2026-10-15
- Electron affinity
- Not reportedPubChem reported gas-phase electron affinity. Sign conventions and electronic states matter; uncertainty is not supplied in this table.PubChem periodic table ↗ · Retrieved 2026-10-15
Isotopes, abundance and decay
Isotopes share a proton count but have different neutron counts. This selection includes naturally tabulated compositions and a reference isotope for other elements. It is not a list of every known isotope.
Explore 6 isotopes of Ca
| Isotope | Relative atomic mass | Natural fraction | Half-life | Decay |
|---|---|---|---|---|
| Ca-40 | 39.962590863(22) | 0.96941(156) | No decay observed | ;2β+ ? |
| Ca-42 | 41.95861783(16) | 0.00647(23) | No decay observed | No decay tabulated |
| Ca-43 | 42.95876644(24) | 0.00135(10) | No decay observed | No decay tabulated |
| Ca-44 | 43.95548156(35) | 0.02086(110) | No decay observed | No decay tabulated |
| Ca-46 | 45.9536890(24) | 0.00004(3) | No decay observed | ;2β− ? |
| Ca-48 | 47.95252276(13) | 0.00187(21) | 56 Ey (uncertainty 10) | ;2β−=?;β− ? |
Fractions use NIST’s terrestrial isotopic compositions, which can vary by sample. Parentheses denote uncertainty in the last digits. Half-lives and decay branches use NUBASE2020; # marks estimates, >/< mark limits. “No decay observed” follows that evaluation and does not prove permanent stability. Decay branch intensities are percentages; SF means spontaneous fission, EC electron capture, IT an isomeric transition.
Follow the science
Use these original references to explore the data and context. Atomic weights can vary with isotope composition; values for elements without stable isotopes may describe a representative isotope rather than a standard atomic weight.