Sulfur (or Sulphur in British English) is the chemical element with the atomic number 16. Its element symbol is S. It is one of the chalcogens, the sixth main group of the periodic table. Of all the elements found in the lithosphere, sulfur is in the 16th place in terms of frequency. Sulfur is a yellow, non-metallic solid that forms a variety of allotropic modifications. In nature, it occurs both in the massive form (i.e. in pure form) and in inorganic compounds (in the latter it mainly occurs as sulfide or sulfate). Certain amino acids and coenzymes also contain sulfur. In organisms, it plays a role in the anaerobic energy production of microorganisms. In the chemical industry, most of the elemental sulfur is used to produce sulfuric acid, which is one of the most technologically significant and most produced basic chemicals. Sulfur oxides, as a component of acid rain, have a significant environmental relevance.