Hydrogen has 3 main isotopes, protiumprotiumA hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. Atomic hydrogen constitutes about 75% of the baryonic mass of the universe.
› wiki › Hydrogen_atomHydrogen atom - Wikipedia deuterium and tritium having mass number 1, 2 and 3 respectively. In the given options, the one which is not an isotope of hydrogen is platinum.