A nonmetal is a substance – particularly, an element – showing none of the properties characteristic of metals. Nonmetals are usually poor conductors of heat and electricity (they are thermal and electrical insulators). Examples include carbon, the halogens, oxygen, nitrogen, phosphorus, and sulfur. The 17 or so nonmetallic elements fill the top right-hand corner of the periodic table. Their atoms are in general relatively small, with nearly-filled electron shells, and have high ionization potentials. They have high electronegativities, and tend to form covalent bonds with each other, and to form anions (negative ions). Nonmetal oxides are acidic or neutral – not basic. See also metalloid.