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

Options

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

Answer

A

Explanation

This phienomenon is associated with the intervention of the `4f` orbitals 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 is lanthanoid contraction. This is because of poor shielding of one of the `4f`-electrons by another in the sub-shell.