What Distinguishes a Metal From a Nonmetal?
In general, metals, unlike nonmetals, are good conductors of heat and electricity, malleable, ductile, and almost always solid at room temperature. Metals very often have only between one and three electrons in their outer valence shells, while nonmetals typically have between four and eight. Metals tend to have a metallic lustre and are opaque as thin sheets, while nonmetallic solids tend to have dull surfaces and are transparent.
Many of the distinctions between metals and nonmetals have notable exceptions. For instance, while all three common states of matter, solid, liquid and gas, are found in nonmetals, almost all metals are solid at room temperature. Mercury, the liquid metal, is the only exception. Meanwhile, while most nonmetals are poor thermal conductors, diamond, a form of pure carbon, is actually the best solid heat conductor in existence. Another form of carbon, graphite, is a good conductor of electricity.
Metals and nonmetals tend to react strongly with each other because of their complementary numbers of electrons in their outer valence shells. For example, an extremely common and vital reaction in nature is the reaction of oxygen with many metals. Indeed, many nonmetals act as electron acceptors, or oxidizers, not just oxygen. Meanwhile many metals readily act as electron donors, or reducers.