Which of the following factor may be regarded as the main cause of lanthanide contraction?

Options

A. Poor shielding of `4` f-electrons in compare to other electrons in the sub-shell.
B. Effective shielding of one of the `4`f-electrons by another in the sub shell.
C. Poorer shielding of `5` d electrons by `4`f-electrons.
D. Greater shielding of `5f` d-electrons by `4` f-electrons.

Answer

A

Explanation

This phenomenon is associated with the intervention of the `4f` orbital which must be filled before the `5d` series of elements begin . The filling of `4f` before `5d` orbital results in a regular decrease in atomic radii called Lanthanoid contraction. This is because of poor shielding of one of the `4f`-electrons by another in the sub-shell.