Atomic nucleus shells indicate that nuclear stability is much more complex than simple closure of shell orbitals with magic numbers of protons and neutrons. For larger nuclei, the shells occupied by nucleons begin to differ significantly from electron shells, but nevertheless, present nuclear theory does predict the magic numbers of filled nuclear shells for both protons and neutrons. The closure of the stable shells predicts unusually stable configurations, analogous to the noble group of nearly-inert gases in chemistry. An example is the stability of the closed shell of 50 protons, which allows tin to have 10 stable isotopes, more than any other
Chronology of the universe universe has cooled to a point where free electrons can combine with the hydrogen and helium nuclei to form neutral atoms. This process is relatively fast (and faster for the helium than for the hydrogen), and is known as recombination. The name is slightly inaccurate and is given for historical reasons: in fact the electrons and atomic nuclei were combining for the first time. Directly combining in a low energy state (ground state) is less efficient, so these hydrogen atoms generally form with the electrons still in a high energy state, and once combined, the electrons quickly release energy in