Question

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

Answer 1

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

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

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

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.

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.)