What Are Isotopes?

Isotopes are variants of a chemical element that share the same number of protons, and therefore the same atomic number and chemical identity, but differ in the number of neutrons in the nucleus, and consequently in their mass number. Because chemical behavior is governed primarily by the electron configuration, which tracks the proton count, isotopes of the same element behave nearly identically in chemical reactions. Their nuclear properties, however, can differ markedly: some isotopes are stable indefinitely, while others are radioactive and decay over time by emitting particles or radiation. As the IAEA explains in its isotope reference materials, there are 254 known stable isotopes and more than 3,000 known radioactive isotopes across the periodic table.

The term "isotope" comes from the Greek for "same place," reflecting the fact that different isotopes of the same element occupy the same position in the periodic table. Notation follows the convention of placing the mass number as a superscript before the element symbol: carbon-12 is written as 12C, and carbon-14 as 14C. Both are carbon, with six protons, but 12C has six neutrons and is stable while 14C has eight neutrons and decays radioactively with a half-life of about 5,730 years.

Stable Isotopes

Stable isotopes do not undergo radioactive decay on any timescale relevant to human observation. Most elements found in nature consist of a mixture of stable isotopes in characteristic natural abundances. Oxygen, for example, is predominantly 16O (about 99.76%) with small amounts of 17O and 18O. These natural variations in isotopic abundance are exploited in stable isotope tracing, where a sample is enriched in a specific isotope and then tracked through a chemical or biological system by mass spectrometry. The National Isotope Development Center coordinates the U.S. supply of stable and radioactive isotopes for research and industry, describing the enrichment and separation techniques that make isotopically pure materials available.

In materials science and engineering, stable isotopes such as deuterium (2H) and carbon-13 are used to label molecules for nuclear magnetic resonance spectroscopy. In nuclear reactors, the choice of isotope composition in fuel and structural materials directly affects neutron moderation, absorption cross-sections, and long-term material integrity.

Radioactive Isotopes

Radioactive isotopes, also called radioisotopes or radionuclides, have unstable nuclei that spontaneously decay by emitting alpha particles, beta particles, gamma rays, or other radiation until they reach a stable configuration. Each radioisotope is characterized by a unique half-life, the time required for half of any given quantity to decay, which ranges from fractions of a second for some artificial isotopes to billions of years for naturally occurring ones such as uranium-238.

The U.S. Department of Energy's isotope program supports production and research on radioisotopes of strategic importance, particularly those needed for medical diagnosis and treatment, national security, and fundamental scientific research. Production methods include neutron irradiation in nuclear reactors, proton bombardment in particle accelerators, and chemical separation of fission products.

Radioactive materials derived from these isotopes require careful handling under regulatory frameworks that govern dose limits, shielding design, and waste disposal, reflecting both their utility and their potential biological hazard.

Applications

Isotopes have applications in a wide range of disciplines, including:

• Nuclear medicine, where short-lived radioisotopes such as technetium-99m are used as tracers in diagnostic imaging and therapy
• Archeology and geology, where radiocarbon dating with carbon-14 and uranium-lead dating with uranium-238 determine the ages of organic materials and ancient rocks
• Nuclear energy, where specific isotopes of uranium and plutonium serve as fission fuels in power reactors
• Industrial radiography and quality control, where gamma-emitting radioisotopes inspect welds and structural components without disassembly
• Scientific research, where both stable and radioactive isotopes label compounds for metabolic studies, drug development, and environmental tracing