Im a uni student and a Phd student asked me in an alpha decay, the helium nuclei is bounded by the strong nuclear force, how do the alpha particle overcome such a strong force and shoot out? And he told me it is because of quantum tunnelling. To my understanding quantum tunnelling is an effect that due to Heisenberg's Uncertainty principle, a particle can have a distribution of energy, which there is a very low but finite probability that a particle can have enough energy to go over the energy barrier. But I then don't understand why only certain isotopes of an element is radioactive since for those stable elements, quantum tunnelling will still take place. Someone could explain to me:D? Thanks!