Nuclides of an element that have the same number of protons but not the same number of neutrons are called isotopes of that element. For example, both uranium-235 and uranium-238 are uranium isotopes, but uranium-235 has 92 protons and 143 neutrons as opposed to uranium-238 which has 92 protons and 146 neutrons.

The stable isotope

Many isotopes are stable. They will not undergo radioactive decay and give off radiation. Other isotopes are not stable. An isotope is stable when there is a balance between the number of neutrons and protons. When an isotope is small and stable, it contains close to an equal number of protons and neutrons. Isotopes that are larger and stable have slightly more neutrons than protons.

Examples of stable nuclides include Hydrogen-1 (which has a nucleus with just one proton) and Carbon-12 (six protons and six neutrons for a total mass of 12).

Top of page

The unstable isotope

When there is an imbalance between protons and neutrons, usually when the ratio of neutrons to protons is too low, the isotope will want to transform itself into a more stable form – a different atom. When this happens, the atom will decrease its mass by ejecting part of its nucleus. It is a spontaneous process that is known as radioactive decay.

There are three main types of radioactive decay:

Alpha decay: Alpha decay occurs when the atom ejects a particle from the nucleus which consists of two neutrons and two protons. When this happens, the atomic number decreases by two and the mass decreases by four.

Beta decay: In basic beta decay, a neutron is turned into a proton and an electron is emitted from the nucleus. The atomic number increases by one, but the mass only decreases slightly.

Gamma decay: Gamma decay takes place when there is residual energy in the nucleus following either alpha or beta decay, or after neutron capture in a nuclear reactor. The residual energy is released as a photon of gamma radiation. Gamma decay does not generally affect the mass and atomic number of the radioisotope.

Top of page


When an isotope disintegrates spontaneously, the excess energy that is emitted is a form of ionizing radiation. In other words, the disintegration gives off radiation and this is called activity. The isotope that changes and emits radiation is called a radioisotope.

Each disintegration is expressed or measured in a unit called the becquerel (Bq). 1 Bq equals one disintegration per second.


Half-life is the time it takes for a radioisotope to decay to half of its starting activity. The symbol is t½.  Each radioisotope has a unique half-life and can be a fraction of a second or billions of years.

For example, iodine-131 takes eight days to reach its half-life, while plutonium-239 takes 24,000 years.

If the original source of the radioactivity is known, how long it takes to decay can be predicted. Similarly, the reverse is true. If the half-life is known, you can identify the radioisotope. The decay is exponential and essentially all radioactivity disappears after about seven half-lives.

Natural versus man-made radioisotopes

Many radioisotopes are naturally occurring. They originated during the formation of the solar system and by interaction of cosmic rays with molecules in the atmosphere. Tritium, for example, is formed by cosmic ray interaction with atmospheric molecules. Some radioisotopes that were formed when our solar system was created have half-lives of billions of years and continue to be present in our environment. Uranium and thorium are examples.

Radioisotopes are produced as a by-product of nuclear reactors and by radioisotope generators such as cyclotrons. Many man-made radioisotopes are used in the fields of nuclear medicine, biochemistry, the manufacturing industry and agriculture.

Top of page

Related Information

Other websites