Chemical element characterized (1) as a good heat and electricity conductor, (2) as opaque but when polished reflects light very well, (3) as malleable, (4) as processable into wires, (5) as sonorous when struck. Cobalt, copper, iron, magnesium, potassium, sodium, and zinc are metals essential to animal, human, and plant health, in trace quantities. Under suitable conditions all metals crystallize. All metals are solid at normal temperature except cesium, gallium, and mercury. Metals tend to form positive ions, being electropositive chemically, and most have strong reactive-affinity with certain non-metallic elements to forms salts, as with chlorine and sulfur.