A substance that consists of atoms of the same type and cannot be broken down into chemically simpler substances. There are 92 naturally occurring elements, although others have been synthesized bringing the known total to more than 100. An element is characterized by its atomic number, which is the total number of protons in the nucleus of each of its atoms. Hydrogen has an atomic number of 1, helium 2, and so on. The complete list of elements, arranged to show similarities in their properties, is known as the periodic table. An element can have different isotopes, which are atoms with different numbers of neutrons in their nuclei. For example, deuterium (1 proton, 1 neutron) and tritium (1 proton, 2 neutrons) are isotopes of hydrogen. In astronomy, all elements heavier than helium are often known collectively as metals.