In the early 1900s, English chemist Frederick Soddy realized that an element could have atoms with different masses that were chemically indistinguishable. These different types are called isotopes — atoms of the same element that differ in mass. Isotopes differ in mass because they have different numbers of neutrons but are chemically identical because they have the same number of protons. Soddy was awarded the Nobel Prize in Chemistry in 1921 for this discovery.

An isotope containing more than the usual number of neutrons is called a heavy isotope. Heavy isotopes tend to be unstable, and unstable isotopes are radioactive. A radioactive isotope is an isotope whose nucleus readily decays, giving off subatomic particles and electromagnetic energy. Different radioactive isotopes (radioisotopes) differ in their half-life, the time it takes for half of any size sample of an isotope to decay.

Radioisotopes emit subatomic particles that can be detected and tracked by imaging technologies. Weakly radioactive isotopes, called radiotracers, with short half-lives, can be used in medical imaging. These are usually eliminated from the body within hours or days via lungs, urine, or stool. Due to the low strength of radiation emitted and shorter half-lives, these radiotracers pose no threat of radiation-induced illness.

Positron emission tomography detects the activity of radioactive glucose, the simple sugar that cells use for energy. The PET camera reveals which tissues of the patient take up the most glucose. The most metabolically active tissues show up as bright "hot spots" on the images. PET can reveal cancerous masses because cancer cells consume glucose at a high rate to fuel their rapid reproduction.

Excessive exposure to radioactive isotopes can damage human cells and even cause cancer and congenital disabilities, but when exposure is controlled, some radioactive isotopes can be useful in medicine. Radiation therapy uses high-energy radiation to damage the DNA of cancer cells, which kills or keeps them from dividing.

This text is partially adapted from Openstax, Chemistry 2e, Section 2.2 Evolution of Atomic Theory, Openstax, Anatomy and Physiology 2e, Section 2.1: Elements and Atoms: the building blocks of matter, and Openstax, Chemistry 2e, Section 21.5: Use of Radioisotopes.