So what is so bad about nuclear chemistry anyways? • In 1902, Frederick Soddy proposed the theory that "radioactivity is the result of a natural change of an isotope of one element into an isotope of a different element." Nuclear reactions involve changes in particles in an atom's nucleus and thus cause a change in the atom itself. • Atoms up to the atomic number 83 stable • Atoms after the atomic number 83 highly reactive and unstable. • Radioisotope - highly reactive and unstable isotopes.