Why are some elements radioactive?
(Lansing State Journal, June 1, 1994)
But more than protons make up the nuclei of atoms. Uncharged particles called neutrons account for more than half of the mass of atoms.
For each element there are different isotopes, atoms with the same number of protons but different numbers of neutrons. For example, carbon atoms always have six protons, but may have six, seven, or eight neutrons. Some isotopes are stable, but others are radioactive.
An isotope will be radioactive if its nuclei are unstable. Large atomic nuclei, with more than 83 protons and their associated complement of neutrons, are inherently unstable. Uranium and plutonium are examples of such elements. Small atomic nuclei may also be radioactive if the ratio of neutrons to protons exceeds certain limits. Even tiny hydrogen, the smallest of atoms, has a radioactive isotope.
When nuclei are unstable, the structure of the nucleus may change with a known probability called its half life. A half life is the period of time it takes half the nuclei present in an isotope to change. They may be very short or very long, depending on the isotope.
When the nuclei change in structure, energy and/or subatomic particles are given off. These occurrences are referred to as radioactive decay.