Question

What is lanthanoid contraction? Actinoid contraction is greater from element to element than lanthanoid contraction. Why?

Hint:

In lanthanoids, the poor shielding by 4f electrons leads to a steady decrease in size. In actinoids, the 5f electrons are even less effective at shielding, leading to a greater contraction.

Answer 1

Lanthanoid contraction describes the progressive reduction in atomic or ionic radii across the lanthanoid series (La to Lu). This phenomenon arises from the inefficient shielding provided by the 4f subshell. As the atomic number increases, new electrons populate the 4f orbitals, failing to adequately shield the nucleus from the growing nuclear charge. Consequently, there is an intensified attraction on the electrons, leading to a diminution of atomic or ionic dimensions.Actinoid contraction is more pronounced than lanthanoid contraction because the 5f subshell in actinoids exhibits even weaker shielding of the nucleus. The 5f orbitals are more spread out and less adept at mitigating the nuclear charge, resulting in a stronger nuclear attraction and, hence, a greater contraction in atomic and ionic radii.Therefore, the actinoid series demonstrates a more substantial decrease in atomic size along the series compared to the lanthanoid series, attributable to the poorer shielding effect of the 5f electrons.