Nonmetals

chemical elements that form simple bodies that do not have properties characteristic of metals. The term “metalloids,” which is sometimes used for nonmetals, is becoming obsolete. Twenty-two elements are classified as nonmetals. Of these, hydrogen, nitrogen, oxygen, fluorine, chlorine, and the inert gases are in the gaseous state at room temperature, bromine is in the liquid state, and boron, carbon, silicon, phosphorus, sulfur, arsenic, selenium, tellurium, iodine, and astatine are in the solid state.

Carbon and sulfur were the only two nonmetals known in antiquity. Arsenic was produced in the 13th century, hydrogen and phosphorus were discovered in the 17th century, and oxygen, nitrogen, chlorine, and tellurium were discovered in the late 18th century. In 1789, A. L. Lavoisier included these nonmetals in a list of simple substances (except chlorine, which was then considered to be oxidized hydrochloric acid). Bromine, iodine, selenium, silicon, and boron were produced in the first half of the 19th century. Fluorine was isolated and the inert gases discovered only toward the end of the 19th century. Astatine was produced synthetically in 1940.

With the exception of the inert gases, which have an extremely stable outer electron shell and are thus virtually chemically inactive, the nonmetals have high electronegativity. Upon adding electrons, atoms of nonmetals form simple anions (for example, Cl^- and O^2-) or, in combinations with other elements, complex anions (for example, ClO₄^-, SO₄^2-, and PO₄^3-).

In D. I. Mendeleev’s periodic system of the elements, the electronegativity of elements increases within the series from the alkali metals toward the right and, within subgroups, from bottom to top. Halogens, which are elements of the VIIa subgroup, are the most electronegative; their electronegativity drops off from fluorine to astatine. With oxygen, nonmetals form acid oxides, which give acids upon addition of water; their strength drops off from subgroup VIIa to IVa.

The simplest compounds of nonmetals with hydrogen are gaseous at room temperature; aqueous solutions of the hydrogen compounds of nonmetals of subgroup VIIa are strong acids. Compounds of nonmetals with one another—for example, chlorine fluoride C1F, iodine chloride IC1, and carbon disulfide CS₂,—are characterized by covalent bonding. In the compounds of typical nonmetals with metals, bonding is predominantly ionic —for example, in potassium chloride, KC1; magnesium oxide, MgO; and aluminum tribromide, AlBr₃.