A chemical element, S, atomic number 16, and atomic weight 32.064. The atomic weight reflects the fact that sulfur is composed of the isotopes ³²S (95.1%), ³³S (0.74%), ³⁴S (4.2%), and ³⁶S (0.016%). The ratios of the various isotopes vary slightly but measurably according to the history of the sample. By virtue of its position in the periodic table, sulfur is classified as a main-group element.