IsotoPes are nuclides with the same number of Protons (that is, pertaining to the same element), but differing numbers of neutrons. These are called the isotopes of an element. [The number of protons in the nucleus is the Atomic Number, Z]. The number of protons (Z) plus the number of neutrons (N) in an isotope gives the Atomic weight (A) of that isotope, which is shown in the top left hand corner of the chemical symbol. Thus Uranium-235 (92p 143n) and Uranium-238 (92p 146n) are both isotopes of uranium.

An element can exist as a variety of different isotopes, some stable against radioactive decay, others unstable and liable to emit various sub-atomic particles at any time. These particles could be protons, neutrons, electrons, positrons, neutrinos, gamma rays, or alpha particles. The element with the greatest number of known isotopes is Xenon with 35. For any given element, new isotopes are continually being found (produced in high energy particle accelerators), usually with shorter and shorter half-lives. Those in the Segre chart left blank are unknown, but if produced, may well have a significant half-life. On the Segre chart, the isotopes lie on a horizontal strip of the chart.

Shown are five isotopes of Oxygen, an inverse beta decayer (positron emitter), O-15; three stable isotopes, O-16, O-17 and O-18; and a beta decayer (electron emitter), O-19.