n+l RULE
So far we have considered the electron configuration of simple atoms. For complicated atoms
which may contain many electrons and have many energy levels or orbitals, the ‘building up’ process
for the electrons is governed by the following rules :
Rule 1. Each electron shell can hold a maximum of 2n2
electrons where n is the shell number.
Rule 2. These electrons are accommodated in s, p, d and f orbitals, the maximum number of
electrons in each type of orbitals being determined by its electron-holding capacity (for
s = 2, p = 6, d = 10 and f = 14).
Rule 3. In the ground state of an atom, the electrons tend to occupy the available orbitals in the
increasing order of energies, the orbitals of lower energy being filled first. This is called
‘building up principle’ or Aufbau Principle (Aufbau is a German expression meaning
building up or construction). Lower energy orbitals are, therefore, better seats for electrons
and better seats are occupied first. 

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