non-metal

Meaning: A nonmetal is an element found mainly in the upper right of the periodic table; hydrogen is also classed as one. Examples are C, N, O, P, S, Se, the halogens and the noble gases. Most nonmetals have high ionization energies and high electronegativities, so they tend to gain electrons and form anions such as Cl⁻ and O²⁻. With other nonmetals they usually share electrons in covalent bonds. As solids they are typically brittle and poor conductors of heat and electricity. Their oxides (CO₂, SO₃) usually react with water to give acidic solutions.

On the MCAT: Expect period and group trends. Moving left to right across a period, atomic radius decreases while electronegativity, ionization energy and electron affinity generally increase, so nonmetals sit at the high end. You may also be asked to classify a compound as ionic (metal + nonmetal) or covalent (nonmetal + nonmetal), or to decide whether an oxide is acidic or basic.

Common confusion: Students assume every nonmetal is a gas or a nonconductor. Position in the periodic table defines the class, not physical state. Bromine is a liquid, carbon and sulfur are solids, and graphite, a form of carbon, conducts electricity. Noble gases are nonmetals too, but they rarely form ions or bonds.