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What is the lanthanoid contraction?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — transition metal Modern version of the periodic table of the elements (printable).

The lanthanoid contraction (also spelled lanthanide contraction) is the steady decrease in the size of atoms and ions of the rare-earth elements as the atomic number increases from lanthanum to lutetium.1

This phenomenon occurs because the 4f electrons added across the series provide very imperfect shielding against the increasing positive charge of the nucleus. Consequently, the effective nuclear charge attracting each electron increases, causing the atomic and ionic radii to shrink.1

In the context of transition metals, the lanthanoid contraction is a critical factor in the chemical similarity between the second and third transition series. Because the 4f orbitals are filled before the third transition series, the valence-shell orbitals of these heavier elements are contracted. This results in elements like hafnium having an atomic radius nearly identical to that of zirconium, leading to an extraordinary similarity in their chemical properties.12