What is "thio-"

Thio- \Thi"o-\ [Gr. ? brimstone, sulphur.] (Chem.) A combining form (also used adjectively) denoting the presence of sulphur. See Sulpho-.

pre. (context chemistry English) containing sulfur / sulphur, especially a compound in which oxygen has been replaced by this element

The prefix thio-, when applied to a chemical, such as an ion, means that an oxygen atom in the compound has been replaced by a sulfur atom.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 R–O–R′, 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 R–S–R′, 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 from Greek theîon = "sulfur" (which occurs in Greek epic poetry as théweion and may come from the same root as Latin fumus ( Indo-European dh-w) and may have originally meant " fumigation substance".)