Which one of the following pairs shows abnormal electronic configuration?

Explanation

The pair of elements that shows an abnormal electron configuration are Cu (copper) and Cr (chromium). Cu has the electron configuration [Ar] 3d10 4s1, which means it has a full 3d orbital with 10 electrons and one electron in its 4s orbital. Cr has the electron configuration [Ar] 3d5 4s1, which means it has a half-filled 3d orbital with 5 electrons and one electron in its 4s orbital. Both of these electron configurations are abnormal in the sense that they do not follow the expected pattern of electron filling based on the Aufbau principle. In general, the 4s orbital is expected to be filled before the 3d orbital, but in these cases, one electron from the 4s orbital of Cu and Cr is transferred to the 3d orbital, resulting in an anomalous configuration. This phenomenon is known as the "exception rule" or the "Aufbau principle exception" and occurs because half-filled or fully-filled orbitals have lower energy and are more stable than partially-filled ones