(i) Isotopes are the atoms of the same elements having the same atomic number but different mass number, e.g. there are 3 isotopes of hydrogen atom, namely protium , deuterium , and tritium .
Uses of isotopes in the field of medicine
(a) An isotope of cobalt (Co-60) is used in the treatment of cancer.
(b) An isotope of iodine (1-131) is used in the treatment of goitre.