# Why Chromium has [Ar] 3d5 4s1 and Copper has [Ar] 3d10 4s1?

Oct 19, 2017

I can give you a summary, but you need to read these links...

It boils down to an increase in exchange energy and decrease in repulsion energy being enough that the $3 d$ orbitals are lower in energy than the $4 s$, but not so low that $\text{Cr}$ can afford to pair one of its electrons up with a $3 d$ electron.

This is easy enough: the $3 d$ orbitals of $\text{Cu}$ are significantly lower in energy than the $4 s$ orbitals of $\text{Cu}$, so electron pairing in the $3 d$ orbitals is more favorable than in the $4 s$ orbital.

These configurations give Cr and Cu better stability than what the electron configurations predicted by the Aufbau principle would.

#### Explanation:

There is a more detailed explanation of this here.

Hope this helps!