Ca2

Ca2+ is the calcium ion, the divalent cation of calcium with a charge of +2. In aqueous solution it is highly hydrated and commonly exists as the hexaaqua complex [Ca(H2O)6]2+. In coordination chemistry it tends to have coordination numbers of 6 to 8 and forms a variety of complexes with carbonate, phosphate, sulfate, and organic ligands, contributing to the formation of minerals such as calcium carbonate and calcium phosphate.

Calcium is abundant in nature; it is a major component of rocks and minerals, and Ca2+ is the predominant dissolved form in many natural waters. It is the principal source of calcium ions in soils, groundwater, and seawater, released by the dissolution of minerals like limestone and gypsum.

Biologically, Ca2+ is essential for many processes. In vertebrates, calcium is a key component of bone and teeth as hydroxyapatite and also acts as a universal intracellular second messenger. Changes in intracellular Ca2+ concentration regulate muscle contraction, neurotransmitter release, enzyme activity, and various signaling pathways. Extracellular Ca2+ helps stabilize cell membranes and participates in blood coagulation and other processes. Typical concentrations differ greatly between compartments: extracellular Ca2+ is around 1–2 millimolar, while intracellular free Ca2+ is maintained at roughly 100 nanomolar.

Regulation of Ca2+ in the body involves parathyroid hormone, calcitonin, and vitamin D to control absorption, excretion, and bone turnover. Cellular homeostasis is maintained by Ca2+-transporting proteins such as Ca2+-ATPases, exchangers, and channels including voltage-gated and receptor-operated types.

In industry and circulation, Ca2+ forms insoluble salts like calcium carbonate and calcium sulfate, and is widely used in cement, lime production, and as a dietary supplement. Excess Ca2+ can be toxic, while deficiency leads to metabolic and bone disorders.