Assignment of elements to classes

The clear-cut nonmetals are hydrogen, nitrogen, oxygen, fluorine, sulfur, chlorine, bromine, and the inert or noble gases (heliiun, neon, argon, krypton, xenon). 

The clear-cut semimetals are boron, silicon, germanium, and tellurium. 

The clear-cut metals constitute all of the remaining elements, with the exception of the problem cases discussed below. 

Carbon. Under ordinary conditions, carbon exists in two common forms, graphite and diamond, of which graphite is the more stable. Diamond is definitely nonmetallic, but graphite has properties on the borderline between a semimetal and a metal. Thus, it is a shiny, black solid, readily separated into flakes, with a metallic conductivity in the plane of the flakes, and a semimetallic conductivity perpendicular to the plane. Other forms include black, petrol-soluble fiillerenes . When pure, these are insulators. 

Phosphorus. This exists in a number of different forms one white, several red, and several black. The white form can be made by condensing the vapour. It melts at 44 °C and boils at 280 A red form is obtained when the white form is heated to just below its boiling point. A black form can be obtained by heating the white form under a very high pressure, or in the presence of a mercury catalyst. The yellow and red forms are insulators the black forms are semiconductors. The most stable form under ordinary conditions is black all forms, however, melt to a colourless liquid. 

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