In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula CxOn−y for some integers x, y, and n. The most common oxocarbon anions are carbonate, CO2−3, and oxalate, C2O2−4. There is however a large number of stable anions in this class, including several ones that have research or industrial use. There are also many unstable anions, like CO−2 and CO−4, that have a fleeting existence during some chemical reactions; and many hypothetical species, like CO4−4, that have been the subject of theoretical studies but have yet to be observed.