Since antiquity, it has been accepted that the materials of nature are made up of a small number of fundamental substances combined in various ways. Today, we call these substances elements. From the seventeenth century through the end of the nineteenth, elements were defined as pure substances that could not be broken down further by the methods of chemistry. In time, it was recognized that each element contains only one type of atom, the basic unit of matter. By 1850, about 60 elements were known, including such common ones as oxygen, copper, gold, and sodium. Today, 118 elements are known. Of these, 94 occur naturally and 24 have been created artificially in the laboratory. Elements are often indicated by a chemical symbol that consists of a one- or two-letter abbreviation of the name of the element. For example, carbon is represented by C, hydrogen by H, and helium by He.