WHy is Copper an exception in electron configuration?
Copper is not an exception except in our minds. The exception lies in our attempt to explain electron configurations. In the case of Cu and Cr for the 3d series, they don't fall into our neat pattern; therefore, we call them exceptions. As to WHY, the best answer I can give is that the
1s2 2s2 2p6 3s2 3p6 3d10 4s1 of copper has a lower energy than any other combination including the one we might expect it to have; i.e.,
1s2 2s2 2p6 3s2 3p6 3d9 4s2. I often see texts say that the 3d orbital has a higher stability when it is half filled (in the case of the other exception of Cr) or completely filled (in the case of Cu). So in the case of Cr, the 3d and 4s are half filled; in the case of Cu the 3d is completely filled and the 4s is half filled. I hope this helps.
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