What is a radioisotope?

Unstable atoms with the same number of protons in the nucleus.

Radioisotopes are radioactive :

• Radioactive means that the nucleus of an atom has an unfavorable combination of neutrons and protons and is therefore unstable.
• Isotopes are atoms that have the same number of protons, but different numbers of neutrons.

A radioisotope is thus a variant of an element that is unstable and will decay by emitting radiation. All stable and radioactive can be found in the table of nuclides. An example:

In this image you can see all the isotopes of the element nitrogen (N). Nitrogen has 7 protons in the nucleus and a variable amount of neutrons. The two black colored isotopes are stable, the colored ones are radioisotopes.

A more general term is radionuclides which refers to all the radioactive elements, these are not necessarily isotopes.