Disulfide

Sulfur can be reduced to a state with oxidation number -2 and form a S^2- ion. This is what happens in most sulfides. It then has the electronic configuration of a noble gas (Argon). In some cases it is only reduced to a state with oxidation number -1. Its configuration then resembles that of chlorine and like Cl is has the tendency to form a bond with another S- ion to form a disulfide (S-S)^2-. Oxygen can also do this e.g. in peroxides: H2O2. The tendency is a bit more elaborate in the case of sulfur. This element can even form polysulfides like (S-S-S-S)^2- etc.