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The thionyl group is SO, a sulfur atom plus an oxygen atom.
It occurs in compounds such as thionyl fluoride, SOF2.
Thionyl chloride, SOCl2, is a common reagent used in organic synthesis to convert carboxylic acids to acyl chlorides.
In organic chemistry, the thionyl group is known as a sulfoxide group or sulfinyl group, and has the general structure RS(=O)R'.
References
Thionyl Wikipedia(Text) CC BY-SA