"each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei" and is radioactive is:

The answer is Isotope. Isotopes; each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element. For example, carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon with mass numbers 12,13 and 14 respectively.