he difference between a stable isotope and an unstable isotope of an element is that . ANSWER the unstable isotope decays and is always transformed into a stable form of the same element the stable isotope has a nucleus that decays spontaneously and is radioactive the unstable isotope has a nucleus that does not decay spontaneously and is never radioactive the stable isotope has a nucleus that decays spontaneously but is not radioactive the unstable isotope has a nucleus that decays spontaneously, is radioactive, and can be transformed into an atom of another elemen