Final answer:
Completely filled d orbitals in subshells increase chemical reactivity.
Step-by-step explanation:
In the case of Cr and Cu, we find that half-filled and completely filled subshells apparently represent conditions of preferred stability. This stability is such that an electron shifts from the 4s into the 3d orbital to gain the extra stability of a half-filled 3d subshell (in Cr) or a filled 3d subshell (in Cu). Other exceptions also occur.
The extra stability of completely filled d orbitals in subshells increases chemical reactivity.
For example, niobium (Nb) is predicted to have the electron configuration [Kr]5s²4d³, but its ground-state electron configuration is actually [Kr]5s¹4d. This is because the electron-electron repulsions experienced by pairing the electrons in the 5s orbital are larger than the energy gap between the 5s and 4d orbitals.