Isotopes What are isotopes? Isotopes are the atoms of the same elements having same atomic number but different mass numbers. As elements are recognized by their atomic number according to the latest theory and also modern period table is based on the atomic number. e.g: Isotopes of Hydrogens. Protium ( 1 H), Protium Deuterium ( 2 H) Deuterium Tritium ( 3 H) Tritium