精选优质文档-倾情为你奉上Unit 1 Metals金属The use of metals has always been a key factor in the development of the social systems of man. Of the roughly 100 basic elements of which all matter is composed, about half are classified as metals. The distinction between a metal and a nonmetal is not always clear-cut. The most basic definition centers around以为中心 the type of bonding existing between the atoms of the element, and around the characteristics of certain of the electrons某种电子 associated w
