Similar to the lanthanoids, actinoids exhibit a gradual decrease in the size of their atoms or $M^{3+}$ ions across the series, a phenomenon known as actinoid contraction. However, the contraction is greater from element to element in the actinoid series compared to the lanthanoid series. This occurs because the $5f$ electrons provide poorer shielding from the increasing nuclear charge than the $4f$ electrons . As a result, the outer electrons are pulled more strongly towards the nucleus, leading to a greater decrease in atomic and ionic sizes.