lanthanide contraction

Lanthanide contraction is a term used in chemistry to describe different but closely related concepts associated with smaller than expected atomic radii of the elements in the lanthanide series (atomic number 58 to 71).
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An effect that causes sixth period elements with filled 4f subshells to be smaller than otherwise expected. The intervention of the lanthanides increases the effective nuclear charge, which offsets the size increase expected from filling the n=6 valence shell. As a consequence, sixth period transition metals are about the same size as their fifth period counterparts.