| As the periodic table of the elements is built up by adding the necessary electrons to match the atomic number, the electrons will take the lowest energy consistent with the Pauli exclusion principle. The maximum population of each shell is determined by the quantum numbers and the diagram at left is one way to illustrate the order of filling of the electron energy states. For a single electron, the energy is determined by the principal quantum n number and that quantum number is used to indicate the "shell" in which the electrons reside. For a given shell in multi-electron atoms, those electrons with lower orbital quantum number l will be lower in energy because of |
The electron configuration for any element may be found by clicking on that element in the periodic table. The first exception to the above scheme that is encountered is chromium, where the fifth 3d electron state is occupied instead of the second 4s state