Question

Why do the electron configurations of chromium and copper seem to disagree with what is expected according to the Aufbau principle?

Answer 1

Aufbau Principle:

• The position of an electron in various energy levels is determined using the Aufbau principle.
• According to this, an atom's electrons are filled in order of increasing energy.
• Before the atomic orbital with high energy, the one with lower energy is filled.

Reason Of Disagreement Between Electronic Configuration of Chromium And Copper:

• The electronic number of Chromium and Copper is $24,29$ respectively.
• The expected electronic configuration of Chromium and Copper is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{4}4s^2$ and $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{9}4s^2$ respectively.
• But these electronic configurations are incorrect.
• According to the Aufbau principle, the subshell that is half-filled or fully filled has greater stability.
• Therefore, in the electronic configuration of Chromium and Copper, if one electron of $4s^2$ comes to the previous shell, it will make $3d^4\text{to}3d^5$ and $3d^9\text{to}3d^{10}$.
• Now the $d$-orbital of Chromium and Copper is half-filled and fully filled and this will increase the stability of the atom.
• So, the correct configuration of Chromium is ${\text{1s}}^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{5}4s^1$ and Copper is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^1$.