An isotope is one of two or more atoms of an element that have the same number of protons, but different numbers of neutrons. As a result, the atomic weight of an isotope is not the same as that of the original element. Isotopes are analogous to ions, but where an ion has gained or lost an electron, an isotope has gained or lost a neutron. Since neutrons have no charge, the net charge of an isotope does not change when an isotope is formed.
In fact, a proper definition of the atomic weight of an element is based on the mass of different isotopes. Many atoms occur naturally as different isotopes; for example, 90.9% of neon atoms on Earth have 10 protons and 10 neutrons, 0.3% have 10 protons and 11 neutrons, and 8.8% of neon atoms have 10 protons and 12 neutrons. Since each of these isotopes has 10 protons and elements have unique numbers of protons, the isotopes are simply different forms of the element neon.Naturally occurring isotopes are taken into account when the atomic weight of an element is calculated, so atomic weight is the average mass of all the isotopes of an element, based on the relative abundance of each isotope. Thus, the atomic weight of neon listed in a periodic table is 20.180 grams per mole (g/mol). Some isotopes are stable and do not gain or lose neutrons but instead maintain a constant atomic mass. Other isotopes, called a radioisotope, are isotopes that emits radiation that can be detected to track the movement of the isotope through a system such as a patient's body. Radioactive decay is the process by which an atom loses energy by emitting radiation. Radioisotopes are incredibly useful tools in biology, both in laboratory research and in medicine. Radioisotopes can be used to "label" a biological molecule, which is called radiolabeling; for example, adenosine triphosphate (ATP) is an important biological molecule of interest to cell biologists. By preparing ATP with phosphorus-32, a radioisotope of phosphorus, scientists can track the movement of ATP through cells using equipment that detects radiation.
Radioisotopes are also useful diagnostic tools in medicine. A type of scan called a positron emission tomography (PET) scan works by detecting radiolabeled molecules that have been injected into a patient. The PET scan produces images that show where the molecules have gone in the body, and the information in the scan can inform the doctor about whether any disease is present. Radioisotopes are also used as treatment in medicine. A common radioactive therapy uses a radioisotope of iodine to treat thyroid disease. Iodine accumulates in the thyroid, so radioactive iodine can selectively destroy diseased thyroid cells while sparing the rest of the cells in the body.