Thio-

{{Short description|Chemical prefix denoting the replacement of an oxygen atom with sulfur}}

{{about|the prefix in organic chemistry||Thio (disambiguation)}}

The prefix thio-, when applied to a chemical, such as an ion, means that an oxygen atom in the compound has been replaced by a sulfur atom.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 {{chem2|R\sO\sR′}}, 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 {{chem2|R\sS\sR′}}, where S is a sulfur atom covalently bonded to two organic groups.{{JerryMarch}} 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 {{ety|grc|θεῖον (theîon)|sulfur}} (which occurs in Greek epic poetry as {{langx|grc|θέ(ϝ)ειον|théweion|label=none}} and may come from the same root as Latin {{lang|la|fumus}} (Indo-European dh-w) and may have originally meant "fumigation substance".)