radioactive decay

Back to SCIENCE main page

The nuclei of some isotopes are unstable. They can split up or ‘decay’ and release radiation. Such isotopes are called radioactive isotopes or radioisotopes. When a radioactive isotope decays, it forms a different atom with a different number of protons.

Changing elements

When an atom emits alpha or beta radiation, its nucleus changes. It becomes the nucleus of a different element. This is because the number of protons in the nucleus determines which element the atom belongs to. These are the changes that occur to the number of particles in an unstable nucleus when it emits a radioactive particle:

In each case, a different element is left behind (though not in the case of gamma radiation).

alpha decay beta decay   gamma decay
change in number of protons
change in number of neutrons

Example 1

Uranium-230 nuclei emit alpha radiation and become nuclei of thorium-226:

Nuclear equation showing uranium-230 undrgoing alpha decay

Remember that an alpha particle is identical to a helium nucleus. Notice that:

  • The mass number goes down by 4.
  • The atomic number goes down by 2.

Example 2

Hydrogen-3 nuclei emit beta radiation and become nuclei of helium-3:

Nuclear equation showing hydrogen-3 undergoing beta decay

Notice that:

  • The mass number stays the same.
  • The atomic number goes up by 1.

[ This page has been adapted from ]