isotopes

atoms that differ from other atoms of the same element because of a different number of neutrons in the nucleus. Isotopes have the same number of protons and therefore the same atomic number but a different mass number (total number of protons and neutrons). Radioactive isotopes decay into other elements or isotopes through the emission of alpha, beta or gamma radiation, and some radioactive isotopes can be produced in the laboratory. RADIOTHERAPY uses such isotopes in the treatment of cancer.