Question

The electron configuration of $Cr$ is (atomic number $=24$):

• A. $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}4p^4$
• B. $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^6$
• C. $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{5}4s^1$
• D. $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}4p^5$

Answer 1

The correct option is C $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{5}4s^1$

Electronic configuration of $Cr$ is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}3d^5$

We might think that it should have been $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^4$, but we should remember that half-filled orbital is more stable than an incompletely filled orbital. d subshell can hold a maximum of $10$ electrons, hence half-filled d subshell will hold $5$ electrons. Thus $d^5$ is more stable than $d^4$.

So, the electronic configuration of Chromium is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}3d^5$.