Upon closer inspection, however, a problem arises with the classical view of alpha decay. Observed kinetic energies of alpha particles in decays are in the range of about 4 to 9 MeV, but the potential barrier of a typical nucleus is about 25 MeV, which means that, assuming that the alpha particle has the same kinetic energy inside the atom as outside, the alpha particle should have no chance for escape from the nucleus. Quantum mechanics, however, allows for a phenomenon known as tunneling, which would give the alpha particle a chance of penetrating the energy barrier, even if its energy were lower than that of the barrier. To illustrate an example in the macroscopic world, a person walking out of a room has a chance of leaving the room by passing straight through the closed door, but this chance is so incredibly small that we never observe such a phenomenon. On the microscopic level, though, energies and masses are much smaller, allowing for such an event to occur with more frequency.
Beta decay is in some ways similar to alpha decay, but in others, the two are completely different.
In beta decay, one of two things happen. Either a neutron breaks down into a proton and an electron which is emitted from the nucleus, or a proton converts into a neutron and a positron which is emitted. In both cases, problems arise. First, the electron or positron emitted carries off much less kinetic energy than it should. Also, neither equation conserves spin, a measure of angular momentum.