AUFBAU’s Principle

Aufbau is a German word meaning to build up. The principle states that, "the filling of electrons in various sub - shell takes place in the increasing order of their energies". Hence lower energy sub-shell are filled up first and successively higher energy sub-shell are filled. The energy of various sub-shell are determined by ( n + l ) rule, where 'n' is principle quantum number and 'l' is Azimuthal quantum number. Lower the value of (n + l ), lower the energy of the sub-shell. In case, the value of ( n + l ) is equal for two sub-shells, the one with lower value of 'n' has lower energy. Thus, the increasing order of energies of the various sub-shells is;

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < 7p and so on.

This can be conveniently remembered by simple diagram as follows;