Actinoid contraction is greater from element to element than lanthanoid contraction. Why?
The actinoid contraction is greater from element to element than lanthanoid contraction due to poor shielding by 5f electrons.
In the actinoid series, as the atomic number increases, the nuclear charge increases. The 5f electrons provide poor shielding to the outer electrons from this increasing nuclear charge. As a result, the effective nuclear charge experienced by the outer electrons increases more significantly, causing a greater decrease in atomic and ionic radii from element to element.
In comparison, the 4f electrons in lanthanoids provide relatively better shielding. Therefore, the contraction observed from element to element is greater in actinoids than in lanthanoids.
Explanation
The textbook explicitly states that 'The contraction is, however, greater from element to element in this series resulting from poor shielding by 5f electrons.' This is the key reason for the greater actinoid contraction. The 5f electrons, due to their diffuse nature, shield the nuclear charge less effectively than the 4f electrons in lanthanoids. Consequently, as we move across the actinoid series, the increasing nuclear charge is felt more strongly by the outer electrons, leading to a more pronounced decrease in size compared to the lanthanoid series.