Question

The electronic configuration in which Aufbau principle is violated is:

\;\;\;\; {\text{ 2s 2p}} $$ A. $[ \uparrow \downarrow ]\;\;\;{\text{ }}[ \uparrow \downarrow {\text{][}} \uparrow {\text{][}} \uparrow {\text{]}} \\\$ B. $[ \uparrow ]\;\;\;{\text{ }}[ \uparrow \downarrow {\text{][}} \uparrow {\text{][}} \uparrow {\text{]}} \\\$ C. $[ \uparrow \downarrow ]\;\;\;{\text{ [}} \uparrow {\text{][}} \uparrow {\text{][}} \uparrow {\text{]}} \\\$ D. $[ \uparrow \uparrow ]\;\;\;{\text{ [}} \uparrow {\text{][}} \uparrow ]{\text{[}} \uparrow {\text{]}} $

Answer 1

As we all know that according to Aufbau principle, the electrons are added one by one into various orbitals in order of their increasing energy of different orbitals starting with the orbitals of lowest energy.

Complete step by step answer:
As we have learnt till now that an atom is built up by filling the electrons in different orbitals. Aufbau principle also informs us about the addition of electrons, one by one into various orbitals in order of their increasing energy starting with the orbitals of lowest energy.
We can calculate the energy of these orbital by using the$(n + l){\text{ }}rule$, where ‘n’ is the principal quantum number which tells us about the electrons shell or level of energy and ‘l’ is the azimuthal quantum number which tells us about the subshell. The orbital with lower value has the lower energy and when two orbitals possess the same energy then the orbital having lower value of n has lower energy.
Remember that the electrons first occupy the lowest energy orbital only when these orbitals are filled, then only the electrons are filled in higher energy orbitals. The order of filling up of electrons in increasing order of energy is as follows: $1s,2s,3s,3p,4s,3d,4p,5s,4d,5p,6s,4f,5d,6p,7s$
In option (A) we can see that, first the lower energy orbital is filled and then only the electron goes to the higher energy orbitals, so it follows the Aufbau principle. Similar is the case in options (C) and (D) and they all follow the Aufbau principle. But in option (B) we see that the s-orbital is not fully filled and yet the electrons went up to the higher energy orbital and filled two electrons which violated the Aufbau principle.

**Therefore, the right answer is (B) option.

Note: **
Always remember the three rules described by Aufbau, Hund’s and Pauli’s exclusion principles. These principles state that electrons will first occupy the lowest energy orbital, if two or more orbitals are of equal energy then the electrons will be singly filled before pairing up and no two electrons can have the same value of all four quantum nos.