Chemistry. The lightest chemical element with the symbol H, atomic number 1, and atomic weight 1. It is found in water and in almost all organic compounds, thus it is a substantial component of all organisms with a central role in production of energy. Its ion (H+), called proton, is the active constituent of all acids in the water system (pH value).
Tritium is the unstable, radioactive isotope, commonly used as marker in biochemical studies.