Metal

A material is called a metal based on the way it reacts to other elements. Metallic elements characteristically form positive ions when their compounds are in solution. Their oxides form hydroxides rather than acids with water. Nearly three-fourths of the elements in each group of the periodic table are metals except for the Group 17 (halogen) and Group 18 (noble gas) elements. Most metals form crystalline solids, and most are good conductors of electricity; most have rather high chemical reactivities. Many metals are quite hard, with high physical strength. When polished, metals tend to be good reflectors of light.

Metals easily form alloys with other metals. The presence of even a small amount of another element in a metal severely affects its properties, as in the case of carbon in iron. Mercury, cesium, and gallium exist as liquids at room temperature.

The behavior of metals as atoms or ions deeply affects the electrochemical reactions they undergo, and similarly affects the metabolism of plants and animals. Iron, copper, cobalt, potassium, and sodium are examples of metals that are essential to biological function. Some metals such as cadmium, mercury, lead, barium, chromium, and beryllium are highly toxic.