The word 'aufbau' in German means 'building up'. The building up of orbitals means the filling up of orbitals with electrons. The principles states: In the ground state of the atoms, the orbitals are filled in order of their increasing energies. In other words, electrons first occupy the lowest-energy orbital available to them and enter into higher energy orbitals only after the lower energy orbitals are filled. The order in which the energies of the orbitals increase and hence the order in which the orbitals are filled is as follows:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s…………….
This order may be remembered by using the method given in Fig. 3.3. Starting from the top, the direction of the arrows gives the order of filling of orbitals. Alternatively, the order of increase of energy of orbitals can be calculated from (n +1) rule, explained below:
The lower the value of (n+1) for an orbital, the lower is its energy. If two orbitals have the same (n+1) value, the orbital with lower value of n has the lower energy.
It may be noted that different subshells of a particular shell have different energies in case of many-electron atoms. However in hydrogen atom, they have the same energy.