In chemistry, after nonmetallic elements such as silicon, chlorine, and helium are classed as either metalloids, halogens, or noble gases, the remaining unclassified elements are hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur and selenium. The nonmetallic elements are sometimes instead divided into two to six (or seven) alternative classes or sets according to, for example, electronegativity; the relative homogeneity of the halogens; molecular structure; the peculiar nature of hydrogen; the corrosive nature of oxygen and the halogens; their respective groups; and variations thereupon.