Question

According to Aufbau principle, the electron has a tendency to occupy the sub-shell which has:
A. Lowest energy
B. Highest energy
C. Average energy
D. No energy

Answer 1

Hint: According to Aufbau principle electrons are filled up in different subshells or orbital in increasing order of their energy. Energy is decided by the “n+l” rule. Aufbau principle can be used to understand the location of electrons in an atom and their corresponding energy levels.

Complete step-by-step answer:
According to the Aufbau rule the subshell or the orbitals with lowest energy levels are occupied before those with higher energy levels.
The energy of an orbital is directly proportional to “n+l”. Here ‘n’ is principal quantum number and ‘l’ is azimuthal quantum number.
If the value of n+l is the same for any of two orbitals and energy level is decided by value of ‘n’, more the value or ‘n’ more is energy.
In subshells energy levels of ‘s’ subshell is the lowest, then ‘p’, then ‘d’, and then ‘f’.
For example, carbon has 6 electrons and if we write its configuration:
For 1s value of ‘n+l’ = 1+0=1, for 2s value of ‘n+l’=2+0=2, for 2p value of ‘n+l’ = 2+1=3. So, energy of 1s < 2s < 2p and then according to Aufbau rule 1st we fill 1s orbital then 2s and then 2p. So, the configuration of carbon is $1{{s}^{2}}2{{s}^{2}}2{{p}^{2}}$.
So, the correct answer is “A”.

Note: The electron configuration of chromium is $[Ar]3{{d}^{5}}4{{s}^{1}}$ and not $[Ar]3{{d}^{4}}4{{s}^{1}}$ (by the Aufbau principle). This exception is because of several factors such as increased stability provided by half filled subshells and relatively low energy gap between the 3d and the 4s subshells.