Metals account for about two thirds of all the elements in the periodic table and have free electrons that distinguish them from nonmetals. Metals have very good strength, ductility, high melting points, thermal and electrical conductivity, and toughness. These properties are due to metallic bonding, which can be thought of as consisting of a three-dimensional array of positively charged ion cores immersed in a cloud of free electrons. The delocalized electrons (free electrons) are responsible for their excellent electrical and thermal properties. Further, the nondirectional character of the interatomic bonding allows plastic (permanent) deformation of the metal with increasing strength for a particular shape and usage. The properties of a metal can be improved by alloying; an alloy is a mixture of a pure metal with one or more other elements. For example, brass is an alloy of copper and zinc metals, while steel consists of iron alloyed with nonmetallic carbon. Alloying can result in three different metallic systems: solid solutions in which one metal in low concentration (solute) substitutes the atomic position of the other (solvent); interstitial solid solution in which a smaller atom, like carbon, ts between the spaces of the existing atom; and intermetallic compound in which a new compound is formed with a new atomic arrangement.