Orbitals are filled according to the Aufbau principle, in order of their energy which is decided by the (n+l) value. Thus, 4s orbital is filled prior to 3d orbital. Hence, the expected outer shell electronic configuration of Cr and Cu are 3d44s2 and 3d94s2 respectively. This is but their actual configurations are 3d54s1 and 3d104s1 respectively. This is because half-filled and fully-filled configurations impart extra stability.