The known elements are classified as metals, nonmetals, or metalloids. Unlike the metals, which are shiny, the nonmetals vary greatly in appearance. Of the approximately 20 elements that are considered to be nonmetals at room temperature: carbon, phosphorous, sulfur, and iodine are solids; bromine is a liquid; hydrogen, oxygen, nitrogen, fluorine, and chlorine are diatomic gases; and helium, neon, argon, krypton, xenon, and radon are monotomic gases.

Nonmetals in the solid state are usually brittle and lack metallic luster. With the exception of graphite, they are poor conductors of electricity. There are wide variations in other physical properties of the nonmetals. For example, helium has a melting point of -452.2°F (-269°C) compared to carbon's melting point of over 6,332°F (3,500°C). Hardness ranges from that of diamond to the softness of white phosphorous. Nonmetals show an even greater variation in chemical properties than do...