After the nonmetallic elements are classified as either noble gases, halogens, or metalloids, the remaining seven nonmetals are hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur, and selenium. In their most stable forms, three are colorless gases (H, N, O); three have a metal-like appearance (C, P, Se); and one is yellow (S). Electrically, graphitic carbon is a semimetal along its planesGreenwood & Earnshaw 2002, p. 277 and a semiconductor in a direction perpendicular to its planes;Atkins et al. 2006, p. 320 phosphorus and selenium are semiconductors;Greenwood & Earnshaw 2002, p. 482; Berger 1997, p. 86 and hydrogen, nitrogen, oxygen, and sulfur are insulators.
