Nonmetal

Nonmetal is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, every element in the periodic table can be termed either a metal or a non-metal. (A few elements with intermediate properties are referred to as metalloids.) The elements generally regarded as nonmetals are:
 * hydrogen (H)
 * In Group 14: carbon (C)
 * In Group 15 (the pnictogens): nitrogen (N), phosphorus (P)
 * Several elements in Group 16, the chalcogens: oxygen (O), sulfur (S), selenium (Se)
 * All elements in Group 17 - the halogens
 * All elements in Group 18 - the noble gases

There is no rigorous definition for the term "nonmetal" - it covers a general spectrum of behaviour. Common properties considered characteristic of a nonmetal include:
 * poor conductors of heat and electricity when compared to metals
 * they form acidic oxides (whereas metals generally form basic oxides)
 * in solid form, they are dull and brittle, rather than metals which are lustrous, ductile or malleable
 * usually have lower densities than metals
 * they have significantly lower melting points and boiling points than metals
 * non-metals have high electronegativity
 * nonmetals usually have little or no luster

Only eighteen elements in the periodic table are generally considered nonmetals, compared to over eighty metals, but nonmetals make up most of the crust, atmosphere and oceans of the earth. Bulk tissues of living organisms are composed almost entirely of nonmetals. Many nonmetals (hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine) are diatomic, and most of the rest are polyatomic.

Metallisation at huge pressures
Nevertheless, even these 20 elements tend to become metallic at large enough pressures (see nearby periodic table at ~300 GPa).