Lanthanoid contraction describes the progressive reduction in size of lanthanoid ions (La$^{3+}$ to Lu$^{3+}$) across the lanthanide series. Even with a higher nuclear charge, electrons are added predominantly to 4f orbitals, which provide insufficient shielding for the increased positive charge. Consequently, the effective nuclear charge experienced by electrons rises, causing a gradual decrease in ionic radius. This contraction is notably pronounced between elements like La and Lu, impacting their properties and those of their compounds, such as the stability of their coordination complexes and atomic dimensions.
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