Atoms of same element with same atomic number but different mass number are isotopes.
(1) Physical properties of the isotopes are different e.g. mass, density.
(2) Chemical properties of the isotopes are same due to same number of electrons.
(1) Uranium isotope is used as a fuel in nuclear reactor (U-235).
(2) Cobalt isotope is used for treatment of cancer (Co-60).
(3) Iodine isotope is used in the treatment of goitre.