Elementary substance is a pure substance made of the same elements. Elementary substance can no longer be broken down into two or more different substances. Such as oxygen (O2), phosphorus (P4), iron (Fe), silver (Ag) and the like. Elementary substance and element are two different concepts, the element is a general term for the same type of atoms of the same nuclear charge number, element is elementary substance when it exists as free state , some elements may form a variety of single substances, such as carbon element which may be form diamond, graphite and amorphous carbon and other elementary substances. According to different natures of elementary substances , they can be divided into metal and nonmetal two categories.