In 1913, Niels Bohr (1885-1962) proposed a model of the hydrogen atom that introduced the concept of quantized energy levels. Bohr assumed that the energy which an electron in an atom may have is quantized, i.e. it can possess only certain amounts of energy. Bohr assumed that electrons rotate around the nucleus in orbits with distances from the nucleus related to a quantum number n. The lowest quantized energy level has n=1; the next highest has n=2, etc. The properties that are predicted by Bohr's model match the experimentally observed values.

Efforts were begun immediately to extend Bohr's model of the hydrogen atom, which has only one electron, to multi-electron atoms.

The electrons of an atom will assume a configuration which has the lowest possible energy, called the ground state. In the hydrogen atom, the single electron is located in the orbit for which...