What is a radioactive isotope of an element referred to as?

A radioactive isotope of an element is referred to as a radioisotope. Radioisotopes have unstable nuclei that release energy in the form of radiation as they decay into more stable forms. This process can lead to the emission of particles such as alpha particles, beta particles, or gamma rays.

The term "isotope" encompasses any variant of an element that has the same number of protons but a different number of neutrons, which can be stable or unstable. However, only those isotopes that exhibit radioactive decay and produce radiation are specifically termed radioisotopes.

Meanwhile, an "ion" refers to an atom or molecule that has gained or lost one or more electrons, resulting in a net electrical charge. The term "element" simply denotes a pure substance consisting of only one type of atom, defined by its atomic number. Thus, the specificity of "radioisotope" is what makes it the correct answer in this context.