An element which exhibits the external characteristics of a metal but behaves chemically both as a metal and as a nonmetal. Arsenic and antimony, for example, are hard crystalline solids that are definitely metallic in appearance. They may, however, undergo reactions that are characteristic of both metals and nonmetals. Certain of their oxides dissolve in either acids or bases, and are said to be amphoteric in character because they behave either as a base or an acid. Many elements form compounds that are amphoteric. However, only when this dualistic chemical behavior is very marked and the external appearance metallic is the element commonly called a metalloid.