什麼是金屬

具有高電導率、高熱導率以及可塑性、延展性和高反光性的物質，在溶解時可以形成正離子和氫氧化物，其氧化物在遇水時並不產生酸。地球上有四分之三的元素屬於金屬，通常質地很硬，呈結晶狀（參閲crystal）固體，能產生多種化學反應，並能同別的金屬一起被製成合金。在元素週期表中，每一個垂直以及從右至左的表格中的金屬元素的價由低向高呈上升趨勢。最常見的金屬有鋁、鐵、鈣、鈉、鉀和鎂，絕大多數都存在於礦石中，而不是遊離狀態。結晶狀的黏着金屬歸結於金屬的黏着性。其原子緊密結合，其周邊活動的電子遍佈在結構中。金屬常被分為如下幾類（但沒有嚴格的區別，有可能出現重複的情況，定義不十分明確）：礆金屬、礆土金屬、過渡元素、貴重金屬（稀有金屬）、鉑金、稀土金屬、錒系金屬、輕金屬和重金屬。許多金屬元素在營養或其他生物化學中扮演重要的作用，通常以微量元素形式存在，在作為元素和化合物時，多數有毒（參閲mercury poisoning和lead poisoning）。

metal

Any of a class of substances with, to some degree, the following properties: good heat and electricity conduction, malleability, ductility, high light reflectivity, and capacity to form positive ions in solution and hydroxides rather than acids when their oxides meet water. About three-quarters of the elements are metals; these are usually fairly hard and strong crystalline (see crystal) solids with high chemical reactivity that readily form alloys with each other. Metallic properties increase from lighter to heavier elements in each vertical group of the periodic table and from right to left in each row. The most abundant metals are aluminum, iron, calcium, sodium, potassium, and magnesium. The vast majority are found as ores rather than free. The cohesiveness of metals in a crystalline structure is attributed to metallic bonding: The atoms are packed close together, with their very mobile outermost electrons all shared throughout the structure. Metals fall into the following classifications (not mutually exclusive and most not rigidly defined): alkali metals, alkaline earth metals, transition elements, noble (precious) metals, platinum metals, lanthanide (rare earth) metals, actinide metals, light metals, and heavy metals. Many have essential roles in nutrition or other biochemical functions, often in trace amounts, and many are toxic as both elements and compounds (see mercury poisoning, lead poisoning).