Radioactive decay of an atom of carbon-14 yields a nitrogen-14 atom. A beta particle of radiation is emitted during the decay process.
Click on image for full size
Original artwork by Windows to the Universe staff (Randy Russell).

Radioactive Decay

Some materials are radioactive. They emit radiation. When an atom of a radioactive substance emits radiation, it is transformed to a new type of atom. This process is called radioactive decay.

There are many types of radiation that can be emitted during radioactive decay. Particle radiation includes alpha and beta particles as well as proton and neutron radiation. Electromagnetic radiation includes high energy gamma rays and X-rays.

Most elements come in various "versions", called isotopes, with different numbers of neutrons and slightly differing properties. In many cases, less-common isotopes of common substances are radioactive. For example, the rare isotope of carbon called carbon-14 is radioactive. It has 8 neutrons (instead of the usual 6) and radiates beta particles. When an atom emits radiation and undergoes radioactive decay, it may be transformed from one isotope to another, or it may become a different element altogether. When carbon-14 decays by emitting a beta particle, it becomes nitrogen-14. Isotopes that do not decay are said to be "stable".

Different radioactive materials take different amounts of time to decay. Scientists use the idea of a half-life to express this. For example, tritium is a radioactive isotope of hydrogen with a half-life of 12.3 years. If we started with 100 kg of radioactive tritium, 12.3 years later we would have just half that - 50 kg! The other half would have become helium-3 via radioactive decay. During the 12.3 years after that, half of the remaining 50 kg of tritium would decay, leaving just 25 kg of tritium. After two half-lives, only one-quarter (half of half) of the original radioactive substance would remain - 25 kg out of the original 100 kg. The half-life of a radioactive material can be very short (less than a second) or very long (thousands of years) or anywhere in between.

Last modified August 26, 2009 by Randy Russell.

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more


Radiation comes in two basic types: electromagnetic radiation transmitted by photons, and particle radiation consisting of electrons, protons, alpha particles, and so forth. Electromagnetic radiation,...more

Particle Radiation

One main type of radiation, particle radiation, is the result of subatomic particles hurtling at tremendous speeds. Protons, cosmic rays, and alpha and beta particles are some of the most common types...more

Electromagnetic Radiation

Electromagnetic radiation is the result of oscillating electric and magnetic fields. The wave of energy generated by such vibrations moves through space at the speed of light. And well it should... for...more

Element (Chemical Element)

An element (also called a "chemical element") is a substance made up entirely of atoms having the same atomic number; that is, all of the atoms have the same number of protons. Hydrogen, helium, oxygen,...more


Isotopes are different "versions" of a chemical element. All atoms of an element have the same number of protons. For example, all hydrogen atoms have one proton, all carbon atoms have six protons, and...more


Carbon-14 is an isotope of the element carbon. All carbon atoms have 6 protons in their nucleus. Most carbon atoms also have 6 neutrons, giving them an atomic mass of 12 ( = 6 protons + 6 neutrons). Carbon-14...more


Physicists use the term "half-life" to describe how long it takes for radioactive materials to decay. When an atom of a radioactive substance decays, it emits radiation and changes into a different type...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA