| Two atoms may have the same number of | | | | Most elements have between two and six stable |
| protons, and thus be of the same element, yet | | | | isotopes. On the other hand, a few elements such |
| differ in their number of neutrons. Such atoms | | | | as technetium have no stable isotopes. Twenty |
| are called isotopes, atoms of the same element | | | | elements, inclusive of gold, fluorine, sodium, |
| having different masses. Thus isotopes are atoms | | | | aluminum, and phosphorus, have only one stable |
| of the same element that have different masses | | | | isotope each. The element with the most stable |
| due to differences in the number of neutrons | | | | isotopes is easy to remember because its name |
| they contain. The name comes from the Greek | | | | is almost the same as its number of stable |
| phrase ‘isos topos’, meaning "same place": | | | | isotopes such as tin, with 10. As for unstable |
| because they have the same atomic number, | | | | isotopes, there are over 1,000, some of which |
| isotopes of the same element occupy the same | | | | exist in nature, but most of which have been |
| position on the periodic table. Because the atomic | | | | created synthetically in laboratories. This number is |
| number of any element is established, sometimes | | | | not fixed; in any case, it is not necessarily |
| isotopes are represented simply with the mass | | | | important, because many of these highly |
| number, thus: 93Ag. They may also be designated | | | | radioactive isotopes last only for fractions of a |
| with a subscript notation indicating the number of | | | | second before decaying to form a stable isotope. |
| neutrons, so that this information can be obtained | | | | Yet radioisotopes in general have so many uses, |
| at a glance without having to do the arithmetic. | | | | in comparison to stable isotopes, that they are |
| For the silver isotope shown here, this is written | | | | often referred to simply as "isotopes." |
| as Isotopes can also be indicated by simple | | | | Before proceeding with a discussion of isotopes |
| nomenclature: for instance, carbon-12 or | | | | and their uses, it is necessary to address a point |
| carbon-13. | | | | raised very often, when it is stated that some |
| Many isotopes are stable, meaning that they are | | | | atoms do have the same numbers of neutrons |
| not subject to radioactive decay, but many more | | | | and protons. In fact, nuclear stability is in part a |
| are radioactive. The latter, also known as | | | | function of neutron-to-proton ratio. Stable nuclei |
| radioisotopes, play a significant role in modern life. | | | | with low atomic numbers (up to about 20) have |
| Carbon-14, for instance, is used for estimating the | | | | approximately the same number of neutrons and |
| age of objects within a relatively recent span of | | | | protons. For example, the most stable and |
| time—up to about 5,000 years—whereas | | | | abundant form of carbon is carbon-12, with six |
| geologists and other scientists use uranium-238 to | | | | protons and six neutrons. Beyond atomic number |
| date minerals of an age on a scale with that of | | | | 20 or so, however, the number of neutrons |
| the Earth. Concerns over nuclear power and | | | | begins to grow: in other words, the lowest mass |
| nuclear weapons testing in the atmosphere have | | | | number is increasingly high in comparison to the |
| heightened awareness of the dangers posed by | | | | atomic number. For example, uranium has an |
| certain kinds of radioactive isotopes, which can | | | | atomic number of 92, but the lowest mass |
| indeed be hazardous to human life. However, the | | | | number for a uranium isotope is not 184, or 92 |
| reality is that people are subjected to considerably | | | | multiplied by two; rather it is 218. The ratio of |
| more radiation from non-nuclear sources. | | | | neutrons to protons necessary for a stable |
| Radioactivity is a term used to describe a | | | | isotope creeps upward along the periodic table: for |
| phenomenon whereby certain materials are | | | | example, tin with an atomic number of 50, has a |
| subject to a form of decay brought about by the | | | | stable isotope with a mass number of 120, |
| emission of high-energy particles or radiation. | | | | indicating a 1.4 to 1 ratio of neutrons to protons. |
| Forms of particles or energy emitted in radiation | | | | For mercury-200, this ratio is 1.5 to 1. |
| include alpha particles (positively charged helium | | | | By definition the higher is the atomic number the |
| nuclei); beta particles (either electrons or | | | | greater is the number of protons in the nucleus. |
| subatomic particles called positrons); or gamma | | | | This means that more neutrons are required to |
| rays, which occupy the highest energy level in the | | | | "bind" the nucleus together. In fact, all nuclei with |
| electromagnetic radiation emitted by the Sun. | | | | 84 protons or more (i.e., starting at polonium and |
| Radioactivity will be discussed below, but for the | | | | moving along the periodic table) are radioactive, |
| present, the principal concern is with radioactive | | | | for the simple reason that it is increasingly difficult |
| properties as a distinguishing factor between the | | | | for the neutrons to withstand the strain of |
| two varieties of isotope. Isotopes are either | | | | keeping so many protons in place. One can predict |
| stable or unstable. The unstable variety, known as | | | | the mode of radioactive decay by noting whether |
| radioisotopes, is subject to radioactive decay, but | | | | the nucleus is neutron-rich or neutron-poor. While |
| in this context, "decay" does not mean what it | | | | neutron-rich nuclei undergo beta emission, which |
| usually does. A radioisotope does not "rot"; it | | | | decreases the numbers of protons in the nucleus, |
| decays by turning into another isotope of the | | | | neutron-poor nuclei typically undergo positron |
| same element or even into another element | | | | emission or electron capture, the first of these |
| entirely. (For example, uranium-238 decays by | | | | being more prevalent among the lighter nuclei. |
| emitting alpha particles, ultimately becoming | | | | Elements with atomic numbers of 84 or greater |
| lead-206.) A stable isotope, on the other hand, has | | | | generally undergo alpha emission, which decreases |
| already become what it is going to be, and will not | | | | the numbers of protons and neutrons by two |
| experience further decay. | | | | each. |