Copper (Cu) is placed in the d-block or transition metals section of the periodic table, even though its last electron enters the 4s subshell. This placement is based on the underlying electron configuration and the behavior of copper in chemical reactions.
The electron configuration of copper is [Ar] 3d¹⁰ 4s². Although the 4s subshell is filled before the 3d subshell, the energy levels of the 3d subshell are very close to the energy levels of the 4s subshell. In certain elements, such as copper, there is a slight energy preference for the 3d subshell to be filled before the 4s subshell.
This preference is due to the stability gained by completely filling the 3d subshell with 10 electrons. The filled 3d subshell provides a higher level of stability compared to having only 9 electrons in the 3d subshell and 2 electrons in the 4s subshell. This is known as the "energy ordering exception" or "Aufbau principle exception."
The placement of copper in the d-block is also consistent with its chemical properties and behavior. Transition metals, including copper, typically exhibit characteristic properties such as variable oxidation states, the formation of colorful complexes, and the ability to catalyze chemical reactions. These properties arise from the partially filled d orbitals, which allow for the formation of complex electron configurations and the involvement of d electrons in bonding.
In summary, although the last electron of copper enters the 4s subshell, it is placed in the d-block due to its overall electron configuration, the slight energy preference for filling the 3d subshell, and its chemical behavior as a transition metal.