Complex sentence: Nuclear transmutation is the conversion of one chemical element or isotope into another, which occurs through nuclear reactions (which an outside particle reacts with a nucleus) or radioactive decay (where no outside particle is needed).
Split into simpler sentences: Nuclear transmutation is the conversion of one chemical element or isotope into another. This occurs either through nuclear reactions (which an outside particle reacts with a nucleus), or through radioactive decay (where no outside particle is needed).