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The lanthanoid contraction is the steady decrease in ionic radii of $Ln^{3+}$ ions from $La^{3+}$ (106 pm) to $Lu^{3+}$ (85 pm), caused by the poor shielding ability of 4f electrons. Each added 4f electron fails to fully screen the increasing nuclear charge, resulting in progressively higher effective nuclear charge and smaller ionic radii.

Major consequences:

(1) 4d-5d element similarity: Elements in the same group from the 4d and 5d series have nearly identical atomic radii. Zr$\frac{160}{Hf}$(159 pm), Nb$\frac{146}{Ta}$(146 pm), Mo$\frac{139}{W}$(139 pm). This makes them "chemical twins" with extremely similar properties. Hf was only discovered in 1923 despite Zr being known since 1789.

(2) Difficulty in separation: All $Ln^{3+}$ ions have similar size, charge, and therefore similar chemical properties. Modern separation uses ion exchange chromatography with chelating eluents.

(3) Decreasing basicity: La(OH)3 (most basic) to Lu(OH)3 (least basic). Smaller ion = more polarising = more covalent M-OH bond = less basic character.

(4) High density of 5d metals: Same size as 4d but much heavier atoms. Os (22.59 g/$cm^3$) and Ir (22.56 g/$cm^3$) are the densest known elements.

(5) Coordination number decrease: Early lanthanoids (La, Ce) can accommodate CN = 9-12; later ones (Yb, Lu) typically show CN = 8-9.