In chemistry, a nonmetal (or non-metal) is a chemical element that generally lacks a predominance of metallic properties; they range from colorless gases (like hydrogen) to shiny solids (like carbon, as graphite). They are usually poor conductors of heat and electricity, and brittle or crumbly when solid, due to their electrons having low mobility. In contrast, metals are good conductors and most are easily flattened into sheets and drawn into wires since their electrons are generally free-moving. Nonmetal atoms tend to attract electrons in chemical reactions and to form acidic compounds.

Two nonmetals, hydrogen and helium, make up about 99 percent of ordinary matter in the observable universe by mass. Five nonmetallic elements, hydrogen, carbon, nitrogen, oxygen and silicon, largely make up the Earth's crust, atmosphere, oceans and biosphere.

Most nonmetals have biological, technological or domestic applications. Living organisms are composed almost entirely of the nonmetals hydrogen, oxygen, carbon, and nitrogen. Nearly all nonmetals have individual uses in medicine, pharmaceuticals, lighting, lasers, and household items.

While the term non-metallic dates from as far back as 1566, there is no widely agreed precise definition of a nonmetal. Some elements have a marked mixture of metallic and nonmetallic properties, and which of these borderline cases are counted as nonmetals varies on the classification criteria. Generally, from 14 to 23 elements are recognized as nonmetals.