# What is the electron configuration of copper? Why is it NOT [Ar]3d^9 4s^2?

Jul 25, 2017

It is $\left[A r\right] 3 {d}^{10} 4 {s}^{1}$, rather than an expected $\left[A r\right] 3 {d}^{9} 4 {s}^{2}$.

And this "anomaly" arises from not recognizing the difference in energy between the $3 d$ and $4 s$ orbitals. For copper, the $3 d$ are about $\text{5.05 eV}$, or about $\text{487.25 kJ/mol}$ LOWER in energy than the $4 s$ (which is a huge difference!).

Hence, it should be clear that the $3 d$ orbitals are filled for copper BEFORE filling the $4 s$.

That gives rise to the so-called "anomalous configuration", which is in fact the correct, normal one when one properly follows the Aufbau principle (lowest energy fills first!):

$\overline{\underline{| \stackrel{\text{ ")(" "[Ar] 3d^10 4s^1" }}{|}}}$

The reason why one would get $3 {d}^{9} 4 {s}^{2}$ is if one filled the $4 s$ first and the $3 d$ second, with the total of $11$ electrons in the $3 d$ and $4 s$ orbitals.

But that wouldn't make sense, since the $3 d$ orbital is not higher in energy, but lower.

(It is just a poor interpretation of the Aufbau principle by inference off of pre-transition metals, and follows from habitually using the periodic table $s$ and $d$ blocks from left to right.)