The isotopes of an element have the same number of protons (or it would not be the same element) and differing numbers of neutrons. Hence the mass number (the number of protons plus neutrons, sometimes shown thus: He-4) is different. Isotopes are either unstable and radioactive, or stable. Hydrogen (the simplest element with just one proton) has three known isotopes: H-1 (protium) with no neutrons, H-2 (deuterium) with one neutron, and radioactive H-3 (tritium) with two neutrons. Isotopes with too few neutrons are unstable, as are those with too many neutrons, only those with about the right number of neutrons (in a ratio of about 1 proton:1 neutron for the light elements) are stable. Those furthest from the stable number have the shortest half-life, some so short they have not been manufactured or discovered; these are un-known. Xenon has the greatest number of known isotopes, numbering 35. The light elements below beryllium have too few protons to have many isotopes, and the heavy elements beyond fermium have too short a half-life for many to have been found. It is thought that elements with atomic number c.113 should have relatively long half-lives.
