Radioactivity and Radioisotopes

After reading this section you will be able to do the following:

  • Define radioactivity and explain how it is produced.
  • Explain how a radioisotope differs from an isotope.

What is radioactivity?

Atoms with unstable nuclei are constantly changing as a result of the imbalance of energy within the nucleus. The unstable nucleus will emit energy in some form and is said to be radioactive. Radioactivity is the release of energy or ejection of matter that results from changes in the nucleus of an atom.

What is a radioisotope?

As discussed in more detail elsewhere, isotopes are variants of an element that, while all having the same number of protons, have differing numbers of neutrons. These variants are called isotopes. Because the like charges of the protons repel each other, these forces are always trying to push the atom’s nucleus apart. The nucleus is held together by something called the binding energy.

In most cases, elements like to have roughly the same number of protons and neutrons because this makes them the most stable. Stable atoms have a binding energy that is strong enough to hold the protons and neutrons together. Even if an atom has an additional neutron or two it may remain stable. However, each additional neutron may upset the balance and cause the atom to become unstable. In an unstable atom, the nucleus changes its configuration by giving off matter of energy to get to a balanced state. As the unstable nucleus changes, it gives off radiation and is said to be radioactive. Radioactive isotopes are often called radioisotopes.

 This is an image of the periodic table. Many of metallic radioisotopes are separated from the rest of the periodic table.

All elements with atomic numbers greater than 83 are radioisotopes meaning that these elements have unstable nuclei and are radioactive. Elements with atomic numbers of 83 and less, have isotopes (stable nucleus) and most have at least one radioisotope (unstable nucleus). As a radioisotope tries to stabilize, it may transform into a new element in a process called transmutation. We will talk about transmutation in more detail a little later.


  1. Radioactivity is the release of energy and matter due to a change in the nucleus of an atom.
  2. Radioisotopes are isotopes that are unstable and release radiation. All isotopes are not radioisotopes.
  3. Transmutation occurs when a radioactive element attempts to become stabilized and transforms into a new element.