Final answer:
Cr and Cu have unusual outer electron configurations, 4s1 3d5 and 4s1 3d10 respectively, due to the relative stability of half-filled (Cr) and fully filled (Cu) d-subshells, influenced by electron-electron repulsion and subshell energy differences.
Step-by-step explanation:
The outer electron configuration of Cr is 4s13d5 and of Cu is 4s13d10 due to the increased stability of half-filled and fully filled d-subshells, respectively. This occurs because an electron from the 4s orbital is promoted to the 3d orbital, leading to a more stable electronic configuration. The stability is attributed to the lower energy and reduced electron-electron repulsions in these configurations. Elements like niobium (Nb) also display such exceptions, highlighting the complexity in predicting electron configurations for elements where subshell energy differences are minimal compared to electron repulsion forces.