Nonmetal

In the context of the periodic table, a nonmetal is a chemical element that mostly lacks distinctive metallic properties. Physically, nonmetals range from colorless gases such as hydrogen to lustrous crystalline solids such as iodine. They are usually less dense than metals and are often poor conductors of heat and electricity. Chemically, they have moderate to very high electronegativity—a tendency to attract electrons in a chemical bond—and their oxides tend to be acidic. Seventeen elements are widely recognized as nonmetals. Additionally, some or all of six borderline elements (metalloids) are sometimes counted as nonmetals. The two lightest nonmetals, hydrogen and helium, together make up about 98% of the mass of ordinary matter in the observable universe. Five nonmetallic elements—hydrogen, carbon, nitrogen, oxygen, and silicon—make up the bulk of Earth’s atmosphere, oceans, biosphere, and crust. Metals, however, are thought to constitute slightly more than half of Earth’s total mass. Chemical compounds and alloys involving multiple elements including nonmetals are widespread. Industrial uses of nonmetals as the dominant component include in electronics, combustion, lubrication and machining. Most nonmetallic elements were identified in the 18th and 19th centuries. While a distinction between metals and other minerals had existed since antiquity, a classification of chemical elements as metallic or nonmetallic emerged only in the late 18th century. Since then about twenty properties have been suggested as criteria for distinguishing nonmetals from metals. In contemporary research usage it is common to use a distinction between metal and not-a-metal based upon the electronic structure of the solids; the elements carbon, arsenic and antimony are then semimetals, a subclass of metals. The rest of the nonmetallic elements are insulators, some of which such as silicon and germanium can readily accommodate dopants that change their electrical conductivity leading to semiconducting behavior.

Definition and applicable elements Unless otherwise noted, this article describes the stable form of an element at standard temperature and pressure (STP).

Nonmetallic chemical elements are often broadly defined as those that largely lack the characteristic properties of metals—namely lustre, malleability, good thermal and electrical conductivity (arising from their electronic band structure), and a general tendency to form basic oxides. There is no widely accepted precise definition based on the extent to which these metallic properties are lacking; any list of nonmetals is therefore open to debate and revision. Fourteen elements are almost always recognized as nonmetals:

Three more are commonly classed as nonmetals, but some sources list them as "metalloids", a term which refers to elements intermediate between metals and nonmetals:

One or more of the six elements most commonly recognized as metalloids are sometimes instead counted as nonmetals:

About 15–20% of the 118 known elements are thus classified as nonmetals.

General properties

Physical

Nonmetals can be generally characterised by their variety in form and color, relatively low density and limited pliability, and generally low melting points and conductivity. Nonmetals show considerable physical diversity, ranging from colourless gases to lustrous solids. Their physical states, densities, mechanical properties, phase-change temperatures, thermal and electrical conductivities, and electronic structures vary widely, largely because of differences in how their atoms or molecules are arranged and bonded. Compared with metals, however, they are generally less dense and, when solid, are brittle or crumbly rather than malleable or ductile.

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