Explain why the ground-state electron configurations for chromium & copper are different from the expected.?

Question

thankz to those who could contribute!

Answer 1

Thats because half filled and fully filled orbitals are highly stable as they are symmetrical...This leads to delocalization of electrons giving extra stability...So for having half filled d orvitals (in case of Cr) and fully-filled orbitals (in case of Cu)....the Auf Bau principle is violated...Also near Cu, the enrgy of 4s and 3d are almost same...so there are other factors that decide whether the electron in question should go into 4s or 3d....One of these factors is symmetry of fully filled d orbitals leading to symmetry...This in turn leads to stability

Answer 2

Or, simply put, the relative energies of the 4s and 3d can change.  When filling the orbitals (Aubfau principle) the 19th electron (K) goes into the lower 4s orbital (Madelung rule).  After the 4s, electrons begin to fill the 3d.

As electrons occupy the 3d the energies of the 4s and 3d become almost the same so that electrons can move back and forth between them.  For chromium the energies are essentially the same and Hund's rule (maximum multiplicity) tells us that electrons at the same energy (degenerate) will singly occupy the orbitals instead of being paired. Therefore an electron "moves" from the 4s to the 3d to give chromium 6 unpaired electrons.  We think of this arrangement as being lower in total energy, and thus more stable.

By the time we get to copper, the 3d is actually lower in energy than the 4s and the lowest energy (and most stable) arrangement is to have a filled 3d and 1 electron in the 4s.