Question

What is the cause of contraction in the atomic size of lanthanoids?

Hint:

The poor shielding by 4f-electrons in lanthanoids causes an increase in effective nuclear charge, resulting in a contraction of atomic size as we move from left to right in the series.

Answer 1

The contraction in the atomic size of lanthanoids is due to the poor shielding effect of the 4f-orbitals. As we move across the lanthanoid series, the 4f-electrons are added, but they do not shield the increasing nuclear charge effectively, leading to a gradual decrease in atomic size.