[Radioactive Isotope] Radioisotopes are atoms with an unstable nucleus, which try to reach a more stable state by emitting radiation. Through one or more steps down its decay chain a more stable number of protons and neutrons is reached. Approximately 3,800 natural and artificial (fission, generator) radioisotopes exist. Radioisotopes are commonly used in science, industry, and nuclear medicine.

See also Isotope, Decay and Decay Chain.

(FDG) Fluorodeoxyglucose F18 is a type of radioactive glucose (sugar). FDG is the most common radioisotope used in PET/CT scanning.

Characterization of atoms by their nuclear properties, as the number of protons and the number of neutrons. The different nuclides of an element are its isotopes (equal proton number, but different neutron numbers). Isomers of this particular nuclide are equal in the proton and mass numbers, but differ in their energy content. Unstable nuclides which are radioactive are called radionuclides.

See also Isotope, Isomer and Decay.

Radioactive decay is the change of instable atoms to a more stable state. This change to a different nuclide by the spontaneous emission of radiation such as alpha or beta particles, gamma rays, or by electron capture follows an element-specific decay chain. Each step in the decay chain has a definite half-life.
Sometimes also the reduction of excitation energy of the nucleus by e.g. internal conversion is mentioned as radioactive decay.

See also Decay Chain, Radioisotope.

Radionuclides are naturally occurring or artificially produced atoms with unstable nuclei. Therefore radionuclides undergo a radioactive decay, and emit gamma ray(s) and/or subatomic particles.
Radionuclides are used as agents in nuclear medicine and to destroy tumors in cancer therapy.

See Isotope and Radioactive Decay.