An element is a substance composed of equal atoms [1] . Naturally occurring elements range from hydrogen (atomic number 1) to uranium (atomic number 92). In artificial nuclear reactions, an additional 26 transuranium elements, all radioactive and short-lived, have so far been discovered [2] . All natural elements occurring on Earth were created in stars [3] . Three elements (iron, oxygen, silicon) make up 80 % of planet Earth's mass, and three elements (oxygen, carbon, hydrogen) make up more than 90 % of all biomass.
More precisely, elements are made up of atoms that have the same number of protons but can have different numbers of neutrons (see Isotopes). With some exceptions (see Molecules, Note 1), elements occur as chemically not further decomposable substances of chemical compounds . They are identified qualitatively and quantitatively with the methods of analytical chemistry.
The six lightest artificially created transuranium elements, ranging from neptunium (93) to californium (98), have subsequently also been discovered to occur naturally on Earth, sometimes as trace elements in uranium ores. The paucity or nonexistence of transuranium elements on Earth is explained by the short half-lives of most of their isotopes.