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 refers to the gradual decrease in atomic or ionic radii across the lanthanoid series from La to Lu. This occurs due to the poor shielding effect of the 4f subshell. As the atomic number increases, the additional electrons are added to the 4f orbitals, which do not effectively shield the nucleus from the increasing nuclear charge. This results in a stronger pull on the electrons, causing the atomic or ionic size to decrease. Actinoid contraction is even greater than the lanthanoid contraction because the 5f subshell in actinoids is even less effective at shielding the nucleus. The 5f orbitals are more diffuse and less capable of shielding the nuclear charge, causing a stronger nuclear attraction and, therefore, a greater contraction in atomic and ionic radii. Thus, the actinoid series shows a larger decrease in atomic size as we move across the series, compared to the lanthanoid series, due to the poorer shielding effect of the 5f electrons.