Hydrogen: The most plentiful element in the universe and one present in all organic compounds. Hydrogen is a gas with an atomic number of 1 and the symbol H.
Two isotopes of hydrogen -- deuterium and tritium -- have been used as tracers in metabolic studies. Tritium is a long-lived weak emitter of radiation and can be easily incorporated in large molecules such as DNA, and is therefore much used for autoradiography in the laboratory.
Antoine Lavoisier coined the name hydrogen from the Greek hydro (water) + genes (forming), reflecting the fact that water is generated by the combustion of this element.