Hydrogen has three isotopes – Protium (1H1), Deuterium (2H1), Tritium (3H1). The number of neutrons is the reason why these isotopes differ from each other in spite of having the same atomic number.
Among these isotopes, the only tritium is radioactive isotope and it emits low energy β particles as it has more number of neutrons.
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Justify the positNCERT - Chemistry Part-II