Question

The correct electronic configuration of $Cu (29)$

• A. $1 s^{2}, 2 s^{2} \,2 p^{6}, 3 s^{2} \,3 p^{6}\, 3 d^{10}, 4 s^{1}$
• B. $1 s^{2}, 2 s^{2}\, 2 p^{6}, 3 s^{2} \,3 p^{5} \,3 d^{11}, 4 s^{1}$
• C. $1 s^{2}, 2 s^{2}\, 2 p^{6}, 3 s^{2} \,3 p^{6} \,3 d^{9}, 4 s^{1}$
• D. $1 s^{2}, 2 s^{2} \,2 p^{6}, 3 s^{2} \,3 p^{5} \,3 d^{10}, 4 s^{2}$

Answer 1

Answer: (A)

The expected electronic configuration of
$Cu (29)$ is as
$Cu (29) 1 s^{2}, 2 s^{2} \,2 p^{5}, 3 s^{2} \,3 p^{6}, 3 d^{9}, 4 s^{2}$
But the actual configuration is as
$Cu (29) 1 s^{2}, 2 s^{2}\, 2 p^{6}, 3 s^{2}\, 3 p^{6}, 3 d^{10}, 4 s^{1}$
This is attributed to the fact that completely filled electronic configuration (ie, $d^{10}$ ) has lower energy. Thus, to acquire increased stability one of the $4 s$-electron goes into the nearby $3 d$-orbital. So, that $3 d$-orbital get completely filled in $Cu$ configuration. The extra stability of completely filled electronic configuration is due to their symmetrical arrangement and large exchange energy.