Aufbau principle

The Aufbau principle (from the German Aufbau, "building up, construction") was an important part of Bohr's original concept of electron configuration. It may be stated as:^[9]

a maximum of two electrons are put into orbitals in the order of increasing orbital energy: the lowest-energy orbitals are filled before electrons are placed in higher-energy orbitals.

The principle works very well (for the ground states of the atoms) for the first 18 elements, then increasingly less well for the following 100 elements. The modern form of the Aufbau principle describes an order of orbital energies given by Madelung's rule, first stated by Erwin Madelung in 1936.^[8][10]

1. Orbitals are filled in the order of increasing n+l;
2. Where two orbitals have the same value of n+l, they are filled in order of increasing n.

This gives the following order for filling the orbitals:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, and 7p

The Aufbau principle can be applied, in a modified form, to the protons and neutrons in the atomic nucleus, as in the shell model of nuclear physics.



Energy Level                         (n+l)Value

1s                                                1
2s                                                2
2p 3s                                           3                      
3p 4s                                           4
3d 4p 5s                                      5
4d 5p 6s                                      6
4f 5d 6p 7s                                  7
5f 6d 7p 8s                                  8

Hint:
Energy level                     Order of filling
1&2                                     s
3&4                                   ps
5&6                                 dps
7&8                                fdps