 Another common mode of nuclear decay is alpha decay. This corresponds to a nucleus emitting a helium-4 nucleus. Since helium nuclei have two protons, a helium-4 nucleus must contain two protons and two neutrons. The helium-4 nucleus is also called an alpha particle. Consider the following three nuclei. Polonium-212 with 84 protons and 128 neutrons. Lid 208 with 82 protons and 126 neutrons. And helium-4 with two protons and two neutrons. Similar to the beta decay process, let us look at the measured atomic masses. Now consider the process where a polonium-212 nucleus spontaneously emits a helium-4 nucleus, leaving behind the Lid 208 nucleus. If we evaluate the total mass of the left-hand side, we can see that it is higher than the total mass of the right-hand side. Hence it is energetically favourable for polonium-212 to emit a helium-4 nucleus, with this alpha particle carrying away kinetic energy to conserve the total energy. Unlike beta decay, in this case the total number of protons is the same on each side, as is the number of neutrons. Again, like beta decay, there needs to be a force responsible for the process. Both the strong nuclear and Coulomb forces are responsible for nuclear alpha decay. We have now looked at both alpha and beta decay. These are two of the three forms of nuclear decay that are named after the first three letters of the Greek alphabet. So let us now talk about the third one, gamma decay.