of the orbitals on n and l are quite complicated but one simple rule is that, the lower the value of ( n + l ) for an orbital, the lower is its energy. If two orbitals have the same value of ( n + l ), the orbital with lower value of n will have the lower energy. The Table . illustrates the ( n + l ) rule and Fig.
. depicts the energy levels of multi- electrons atoms. It may be noted that different subshells of a particular shell have different energies in case of multi-electrons atoms. However, in hydrogen atom, these have the same energy.
Lastly it may be mentioned here that energies of the orbitals in the same subshell decrease with increase in the atomic number ( Z eff ) . For example, energy of s orbital of hydrogen atom is greater than that of s orbital of lithium and that of lithium is greater than that of sodium and so on, that is, E s (H) > E s (Li) > E s (Na) > E s (K). . .
Filling of Orbitals in Atom The filling of electrons into the orbitals of different atoms takes place according to the aufbau principle which is based on the Pauli’s exclusion principle, the Hund’s rule of maximum multiplicity and the relative energies of the orbitals. Aufbau Principle The word ‘ aufbau ’ in German means ‘building up’. The building up of orbitals means the Table . Arrangement of Orbitals with Increasing Energy on the Basis of ( n+l ) Rule filling up of orbitals with electrons.
The principle 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. As you have learnt above, energy of a given orbital depends upon effective nuclear charge and different type of orbitals are affected to different extent. Thus,