Mustard gas is bis(2-chloroethyl) sulphide. It's chemical structure is as follows:
Sulphur mustard is also known as "mustard gas or mustard agent," or by the military designations H, HD, and HT. Sulphur mustard sometimes smells like garlic, onions or mustrad and sometimes has no colour. It can be a vapours (the gaseous form of a liquid), an oily-textured liquid, or a solid.