What Is an Isotope?

Every atom of an element has the same number of protons in its nucleus, but the number of neutrons can vary — each variant is called an isotope. Carbon, for example, has three natural isotopes: carbon-12 (6 neutrons, 98.9% of all carbon), carbon-13 (7 neutrons, ~1.1%), and radioactive carbon-14 (8 neutrons, trace amounts). Frederick Soddy coined the term "isotope" in 1913, from the Greek for "same place" because isotopes occupy the same position on the periodic table. Stable isotopes are used in medical imaging, geological dating, and forensic analysis, while radioactive isotopes (radioisotopes) power applications from cancer treatment to smoke detectors. Carbon-14 dating, developed by Willard Libby in 1949, revolutionised archaeology by allowing organic materials up to ~50,000 years old to be dated.