Question

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

Answer 1

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

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Chromium is explained in great detail here

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

Copper is explained over here.

This is easy enough: the `3d` orbitals of `"Cu"` are significantly lower in energy than the `4s` orbitals of `"Cu"`, so electron pairing in the `3d` orbitals is more favorable than in the `4s` orbital.

Answer 2

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!