Here's why:
* Inner Transition Metals: These elements are characterized by the filling of the f orbitals, which are located inside the d orbitals and shield them from the outer electrons.
* 4f and 5f orbitals: These orbitals are the highest energy levels available in the respective periods. The final electron enters these orbitals, leading to the unique properties of these elements.
Example:
* Lanthanum (La), the first lanthanide, has its final electron entering the 5d orbital, but subsequent lanthanides fill their 4f orbitals.
* Actinium (Ac), the first actinide, has its final electron entering the 6d orbital, but subsequent actinides fill their 5f orbitals.
Therefore, the final electron in inner transition metals always goes into the f-subshell that corresponds to the period they are in.
