Poor shielding of nuclear charge by d or f-orbital elements is responsible for which of the following facts?

The nucleus charge is not well-shielded by the d and f orbitals. Therefore, right after the d or f orbitals have been fully filled, there is a considerable reduction in the size of the ions. The term for this is lanthanide contraction. Due to lanthanide contraction, the atomic radii of $Nb\left(N{b}^{3+}=0.72\dot{A}\right)$ and $\mathrm{Ta}\left(T{a}^{3+}=0.72\text{Å}\right)$ are practically similar. This explains why gold has a higher ionisation energy than silver.