CO32

The carbonate ion, CO3^2− (often represented in plain text as CO32−), is a triatomic inorganic anion that carries a −2 charge. It is the conjugate base of carbonic acid and a major component of carbonate rocks and many dissolved inorganic carbon species.

Structure and bonding: CO3^2− has a trigonal planar geometry with a central carbon atom bonded to three oxygen atoms. The three C–O bonds are equivalent due to resonance, and the ion belongs to the D3h point group. The average C–O bond length is about 1.29 Å. The carbonate ion serves as a weak base, readily accepting protons to form bicarbonate.

Occurrence and formation: CO3^2− forms by the dissolution of carbon dioxide in water followed by deprotonation: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3−; HCO3− ⇌ H+ + CO3^2−. It is abundant in natural waters, contributing to the carbonate buffering system that stabilizes pH. In geology, carbonate minerals such as calcite (CaCO3) and dolomite (CaMg(CO3)2) are major constituents of sedimentary rocks.

Environmental significance: In oceans and freshwater systems, CO3^2− concentration is pH-dependent and influences the capacity for calcification by marine organisms. Rising atmospheric CO2 lowers pH and reduces CO3^2− availability, impacting shell-forming species and long-term carbon cycling.

Applications and safety: Carbonate minerals are widely used in cement, construction, and industrial chemistry (for example, soda ash, Na2CO3). While CO3^2− itself is not highly toxic, carbonate salts can contribute to water hardness and ecological effects at high concentrations.