Nuclear decays
For a nucleus of a given size, the addition of extra nucleons usually lowers the overall energy. The willingness of nucleons to coalesce differs from the willingess of common liquids to coalesce in two different aspects. First, since the attraction is only strong enough to bind when the interaction is between protons and neutrons and not between protons and protons or neutrons and neutrons. This pushes nuclei towards the N = Z condition. Secondly, Coulomb repulsion makes very large nuclei unstable. Thus, the most energetically favorable nucleus is not the largest one, as it would be with a water droplet, but is instead 56Fe.
Nuclei much lighter than iron can undergo fusion to form more energetically favorable nuclei and release energy. However, due to Coulomb repulsion, nuclei rarely get close enough to one another to fuse. Nuclei that are much larger than iron can lower their energy through fission. Nuclei that wish to attain a more favorable ratio of neutrons to protons can undergo beta decay. Beta decay is so named because the high-energy electrons observed from early beta decays, were not initially recognized as electrons and were called beta rays. The simplest example of a beta decay is the decay of a neutron to a proton, an electron and an anti-neutrino. A neutrino is a nearly massless (possible massless) particle which is neutral and very difficult to detect as it can travel through a solar system's length of lead and not interact. Another form of decay is alpha decay which involves the emission of an alpha particle (which is a helium nucleus) from a nucleus. Alpha decays are common in very heavy nuclei.
Although nuclei and other particles can change (e.g. beta decay of a neutron) the reactions must satisfy 3 conservation laws.
Baryon number conservation: Protons and neutrons are examples of baryons. Electrons, photons and neutrinos have no baryon number. The number of baryons must be the same before and after the reaction. Anti-protons and anti-neutrons have a negative baryon number. Therefore a proton and an anti-proton could decay into two photons.
Lepton number conservation: Electrons and neutrinos are examples of leptons. Anti-electrons and anti-neutrinos are equivalent partices with negative lepton number. Photons, protons and neutrons have zero lepton number.
Electric charge conservation:. Protons have a positive charge, electrons have a negative charge and neutrons, photons and neutrinos are neutral. An anti-proton has a negative charge and an anti-electron (positron) has a positve charge).
All three of these conservation laws are used in the beta decay of a neutron,
Baryon number is conserved by the presence of the proton on the right. The electron is necessary to conserve electric charge and the anti-neutrino (the 'anti" being noted by the overbar) ensures the conservation of lepton number. More about the conservation laws will be stated when we cover particle physics.