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Question: Why is the electronic configuration of \(F{e^{ + 2}}\) not \([Ar]4{s^2}3{d^4}\) but \([Ar]3{d^6}\) ?...

Why is the electronic configuration of Fe+2F{e^{ + 2}} not [Ar]4s23d4[Ar]4{s^2}3{d^4} but [Ar]3d6[Ar]3{d^6} ?

Explanation

Solution

This question can be answered by knowing the Aufbau principle or also by considering the stability of the atom obtained by the stable electronic configuration. The Aufbau principle states that the orbital with lower energy is filled first. We will discuss this further in the question.

Complete answer:
The electronic configuration of Fe is found to be: 1s22s22p63s23p63d104s21{s^2}2{s^2}2{p^6}3{s^2}3{p^6}3{d^{10}}4{s^2}
The Aufbau principle that originates from the Pauli’s exclusion principle states that no two electrons can have all sets of quantum numbers the same, hence they have to pile up or build up into higher energy levels. According to this principle, the orbitals having lower energies are filled up first before the higher energy orbitals. When there are orbitals of the same energy they tend to singly fill the orbitals.
It is found that the energy of 3d orbitals is slightly greater than that of 4s orbital. Which means that 4s will fill first than 3d. Hence while filling the orbitals, the electrons enter into 4s first. After 4s, 3d orbitals are filled, followed by 4p orbital. The same, minute differences in energy occur for the filling of 4f being only after that of 6s.
This exception is only followed for the building up or filling up the orbitals, whereas the 4s orbital is only considered as the outermost orbital having highest energy. When the ionization of the atom takes place, the electrons are lost from the 4s orbital itself, which is the most shielded orbital. Due to the electrons filling in the 3d orbital, the 4s orbital becomes massively shielded which causes the electrons of 4s to get removed easily, making it the valence shell.
Because of this reason, the electronic configuration of Fe+2F{e^{ + 2}} is [Ar]3d6[Ar]3{d^6} (since electrons are lost from 4s and not 3d).

Note:
Remember that irrespective of how the electrons are filled in the orbitals, the removal of electrons always occurs from the orbital having highest principal quantum number (n) . If the quantum numbers are same, then the electrons are removed from the orbital which has higher value of (n+l)(n + l)