The prefix thio-, when applied to a chemical, such as an ion, means that an oxygen atom in the compound has been replaced by a sulfur atom. This term is often used in organic chemistry. For example, from the word ether, referring to an oxygen-containing compound having the general chemical structure , where R and R′ are organic functional groups and O is an oxygen atom, comes the word thioether, which refers to an analogous compound with the general structure , where S is a sulfur atom covalently bonded to two organic groups. A chemical reaction involving the replacement of oxygen to sulfur is called thionation or thiation. Thio- can be prefixed with di- and tri- in chemical nomenclature. The word derives (which occurs in Greek epic poetry as and may come from the same root as Latin (Indo-European dh-w) and may have originally meant "fumigation substance".) Examples Thioamide Thiocyanate Thioether Thioketone Thiol Thiophene Thiourea Thiosulfate See also Organosulfur compounds IUPAC nomenclature of organic chemistry References Chemistry prefixes Prefixes