What is the electron configuration of copper? Why is it NOT [Ar]3d^9 4s^2[Ar]3d94s2?

1 Answer
Truong-Son N.
Jul 25, 2017

It is [Ar] 3d^10 4s^1[Ar]3d104s1, rather than an expected [Ar] 3d^9 4s^2[Ar]3d94s2.

And this "anomaly" arises from not recognizing the difference in energy between the 3d3d and 4s4s orbitals. For copper, the 3d3d are about "5.05 eV"5.05 eV, or about "487.25 kJ/mol"487.25 kJ/mol LOWER in energy than the 4s4s (which is a huge difference!).

Hence, it should be clear that the 3d3d orbitals are filled for copper BEFORE filling the 4s4s.

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!):

barul(|stackrel(" ")(" "[Ar] 3d^10 4s^1" ")|)

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

![http://ptable.com/](useruploads.socratic.org)

But that wouldn't make sense, since the 3d 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.)

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