The lanthanoid contraction is caused by imperfect shielding of one electron by another in the same sub-shell. While the decrease in atomic radii is not quite regular, M³⁺ ions exhibit a more regular contraction. This is attributed to the imperfect shielding being less effective for 4f electrons compared to 3d electrons.
What causes the lanthanoid contraction, and how does it differ in its effects on atomic radii and M³⁺ ions?
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The lanthanoid contraction arises from poor shielding of inner 4f electrons. While lanthanoids fill 4f orbitals, the inefficient screening of outer electrons results in an incomplete shielding of the increasing nuclear charge, causing a smaller-than-expected increase in atomic and ionic radii. In atomic radii, the lanthanoid contraction leads to similar sizes among consecutive elements. In M³⁺ ions, where outer electrons are lost, the contraction is less significant as the influence of the inner electrons diminishes. Thus, M³⁺ ions of consecutive lanthanoids exhibit less variation in ionic radii compared to their atomic radii, highlighting the specific impact on ion sizes.