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An atomic species is defined by two whole numbers: the number of protons in the nucleus (known as Z, or atomic number) and the total number of protons plus neutrons (known as Z, or mass number).

Isotopes are the atoms in an element that have the same atomic number but a different atomic mass; that is, the same number of protons and thus identical chemical properties, but different numbers of neutrons and consequently different physical properties. Isotopes can be stable or unstable or radioisotopes. In the latter, their nuclei have a special property: they emit energy in the form of ionizing radiation while searching for a more stable configuration.

Radioisotopes are radioactive isotopes of an element. Different isotopes of the same element have the same number of protons in their atomic nuclei but differing numbers of neutrons. They can also be defined as atoms that contain an unstable combination of neutrons and protons. These are radioactive isotopes, since they have an unstable atomic nucleus (due to the balance between neutrons and protons) and emit energy and particles when it changes to a more stable form. The energy liberated in the form change can be measured with a Geiger counter or with photographic film. Each radioisotope has a characteristic disintegration or semi-life period. Energy may be liberated mostly in the form of alpha (helium nuclei), beta, (electrons or positrons), or gamma (electromagnetic energy) rays.

Several unstable and artificial radioactive isotopes have medical uses. For instance, a technetium isotope (99mTc) may be used to identify blocked blood vessels. Various natural radioactive isotopes are used to determine chronologies, such as the archeological kind (14C).

Naturally Occuring Radioisotopes

Natural isotopes are either stable isotopes, radioactive isotopes that have a sufficiently long half-life to allow them to exist in substantial concentrations in the Earth (e.g. bismuth-209, with a half-life of 1.9 * 1019 years, potassium-40 with a half-life of 1.251(3)×109 y), daughter products of those isotopes (e.g. 234Th, with a half-life of 24 days) or cosmogenic elements. The heaviest stable isotope is lead-208, but the heaviest ‘natural’ isotope is U-238.

Many elements have both natural and artificial isotopes (e.g. hydrogen has three natural isotopes and another four known artificial isotopes).

A further distinction among stable natural isotopes is division into primordial (existed when the Solar System formed) and cosmogenic elements. Natural isotopes must be either stable, have a half-life exceeding about 7*107 years (there are 34 isotopes in this category, see stable isotope for more details) or are generated in large amounts cosmogenically (e.g. 14C, which has a half life of only 6000 years but is made by cosmic rays colliding with 14N)

Some radioisotopes occur in nature with a half-life less than 7*107 years (carbon-14 5,730 ± 40 years, tritium 12.32 years etc.). They are synthesed all the time because of cosmic radiation. For carbon-14 this fact has practice value because it is used for radiocarbon dating.

Artificially produced radioisotopes

All artificial isotopes of elements with atomic numbers of 83 or higher are radioactive. All elements heavier than barium, which has the atomic number 83, are naturally radioactive or only have one stable isotope. Elements with atomic numbers below 83 can occur as either stable or unstable isotopes.

Artificial isotopes are used in physics research, medicine and as industrial radiation sources. Within medicine, they are used as tracer compounds and to treat cancer and other diseases. Technetium-99 and iodine-131 are examples of artificial isotopes used in medicine. Technetium is used as a tracer in diagnostic tests, and iodine is used to treat thyroid cancer.

Chromium-51 is used to label red blood cells and quantify gastro-intestinal protein loss. Iodine-131 is Used to diagnose and treat various diseases associated with the human thyroid. Iridium-192 is for use as an internal radiotherapy source for certain cancers, including those of the head and breast. Molybdenum-99 66 is used as the ‘parent’ in a generator to produce technetium-99.

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