CO3

CO3 is the carbonate group, most commonly encountered as the carbonate ion CO3^2−. It is an oxoanion formed by one carbon atom bonded to three oxygen atoms, and it carries an overall -2 charge. Due to resonance among the C–O bonds, each carbon–oxygen bond has partial double-bond character, giving a trigonal planar geometry for the central carbon.

In aqueous solution, carbonate exists in a dynamic set of equilibria with carbon dioxide and carbonic acid. The key couples are H2CO3 ⇌ HCO3− + H+ (pKa about 6.3–6.5) and HCO3− ⇌ CO3^2− + H+ (pKa about 10.2–10.3). Thus, the relative abundance of CO3^2− depends strongly on pH: at neutral to slightly basic conditions, bicarbonate dominates; at higher pH, carbonate becomes more significant. In seawater, CO3^2− is present in smaller amounts relative to HCO3−, contributing to the carbonate buffering system that regulates ocean pH.

CO3^2− forms many salts with metals, known as carbonates, such as calcium carbonate (CaCO3), sodium carbonate (Na2CO3), and magnesium carbonate (MgCO3). It readily reacts with acids to release carbon dioxide and water. Beyond its role in geology and chemistry, carbonate is involved in biological mineralization, soil chemistry, and industrial processes. Its applications range from cement and construction materials to glass production and various buffering and pH-control contexts.