Atomic nuclei are composed of protons and neutrons.
Some isotopes exhibit radioactivity, and over a period decay into an isotope of a different element.
Knowing the rate of decay one can sometimes calculate the age of an object from the proportion of isotopes present in it. This is the basis of the radiometric dating . Radioactive isotopes are important technologically, and many are produced by artificial means for use in medicine, weaponry, and the nuclear power industry. The Penguin Dictionary of Psychology 3. All rights reserved.