Copper orbital filling order

Why does the orbital filling order given by expression (8.22) fail for chromium and copper ... 

The periodic table tells us that five hd orbitals are next available for electron occupancy. The next three elements fill in order, as predicted, to vanadium (V, Z = 23) [Ar]4 3 i. Chromium (Cr, Z = 24) is the first element to break the orderly sequence in which the lowest-energy orbitals are filled. Its configuration is [Ar]4 3fi, rather than the expected [Ax]As l>d. This is generally attributed to an extra stability found when all orbitals in a sublevel are half-filled or completely filled. Manganese (Mn, Z = 25) puts us back on the track, only to be derailed again at copper (Cu, Z = 29) [Ar]4 3d °. Zinc (Zn, Z = 30) has the expected configuration [Ar]4 3(i °. Examine the sequence for atomic numbers 21 to 30 in Figure 11.15 and note the two exceptions. 

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