Elements composed of atoms with unstable nuclei are called radioactive elements or radioelements, while the radioactive isotopes of these and other elements are called radionuclides or radioisotopes.

Some radioisotopes exist in nature:

	carbon-14 (C-14), created in the upper atmosphere, forms part of all living matter;
	in one ton of earth, there is on average: 5 g of potassium-40 (K-40), 2.5 g of uranium-238 (U-238), 10 g of thorium-232 (Th-232).

There are also elements such as radon which are generated by the disintegration of uranium and thorium.

Other radioisotopes are produced artificially, mainly in the cores of nuclear power plants. They include: strontium-90 (Sr-90), iodine-131 (I-131), caesium-137 (Cs-137), neptunium-237 (Np-237), plutonium-239 (Pu-239).