Chemical element · reference page

20Ca

Calcium

Calcium is element 20 (Ca), located in period 4, group 2 and the s block. At room temperature its reference phase is solid. Its electron configuration is [Ar] 4s2.

Atomic number20

Standard atomic weight40.078(4)

Group2

Period4

Blocks

Electron configuration[Ar] 4s2

Electronic structure

What controls its chemistry

The outer configuration ends in s². Removing two valence electrons produces a +2 ion, which is the characteristic ionic state of the alkaline-earth metals.

Compared with group 1 metals, the two-electron outer shell changes ionisation energies, bonding and reactivity. Their compounds commonly occur as oxides, carbonates, sulfates or halides rather than as free metals in nature.

Chemical forms

Compounds, ions and materials

The +2 oxidation state dominates. Oxides, hydroxides, halides, carbonates and sulfates are common families of calcium compounds, although their solubility and thermal stability vary systematically down group 2.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 1.55 g/cm³. The melting point is 1 115 K (841.9 °C). The boiling point is 1 757 K (1 484 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where calcium is found and used

In practice, calcium is encountered in bones and teeth, cement, lime and biology.

In practice

Examples of real uses

Bones and teeth

Calcium-rich mineral phases provide hardness and structural support in bones and teeth.

Cement

Calcium compounds participate in the mineral phases or reactions that give cement and concrete their properties.

Lime

Calcium compounds are central to lime manufacture and to reactions used in construction, metallurgy and environmental treatment.

Biology

Calcium ions or compounds participate in biological structures, enzymes, signalling or metabolism.

Periodic position

What its neighbours tell us

Calcium is in period 4 and group 2. In the neighbourhood shown here, it lies between potassium and scandium in the same period; magnesium is above it and strontium below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationalkaline earth metal

Phase at room temperatureSolid

Electron shells2 · 8 · 8 · 2

Density1.55 g/cm³

Melting point1 115 K (841.9 °C)

Boiling point1 757 K (1 484 °C)

Pauling electronegativity1

First ionisation energy589.8 kJ/mol

Electron affinity2.37 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of calcium is associated with Humphry Davy. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is 40.078(4). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Calcium has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The hazards associated with calcium depend on chemical form, dose and route of exposure. Pure calcium, its ions and its compounds can have very different biological and environmental effects, so safety data should be checked for the actual substance being handled.