The atoms of the same element may have different numbers of neutrons in the nucleus. Although this difference in number does not affect the property of the atom, the weight of the atoms may differ (Re-member, neutrons are of the same mass and size as protons.). Hydrogen, for example, may have a proton and no neutrons or one neutron or two neutrons in the nucleus. The atoms of an element that has a dif-ferent number of neutrons in the nucleus are known as isotopes. Isotopes are referred to by the combined number of protons and neutrons (i.e., mass num-ber).
The mass number is the number of protons andneutrons in an atom. In the above example, hydrogen with one proton is hydrogen-1 (1H); with one proton and one neutron, hydrogen-2 (2H); and with one pro-ton and two neutrons, hydrogen-3 (3H).
Some of the isotopes of certain elements contain nuclei, which spontaneously emit subatomic parti-cles known as radioisotopes.Radioisotopes are said to be radioactive. These emissions can be dangerous as they can damage or destroy cells and exposure to these emissions increases the risk of cancer. In medi-cine, radioisotopes are used for medical imaging and destroying cancerous cells.