Together with the metals and metalloids, a nonmetal is one of three categories of chemical elements as distinguished by ionization and bonding properties. These properties stem from the fact that nonmetals are highly electronegative, i.e. they gain valence electrons from other atoms more readily than they give them up.
The nonmetals are, in order of atomic number:
There are only twelve known nonmetals, compared to over eighty metals, but nonmetals make up most of the earth, particularly in the outer layers. 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 otherwise polyatomic.