hydrogen

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.

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