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:

• hydrogen (H)
• carbon (C)
• nitrogen (N)
• oxygen (O)
• fluorine (F)
• phosphorus (P)
• sulfur (S)
• chlorine (Cl)
• selenium (Se)
• bromine (Br)
• iodine (I)
• astatine (At)

Most nonmetals are found at the upper right of the periodic table. The exception is hydrogen, which is usually placed at the upper left with the alkali metals, but behaves like a nonmetal under most conditions. Unlike metals, which are electrically conductive, a nonmetal may be an insulator or a semiconductor. Nonmetals may form ionic bonds with metals by gaining electrons, or covalent bonds with other nonmetals. The oxides of nonmetals are acidic.

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.