Lanthanoide Contraction

Each succeeding lanthanoide differs from its immediate predecessor in having one more electron in the 4f orbitals (except for some exceptions) and one extra proton in the nucleus of the atom.

The 4f electrons constitute inner shells and are ineffective in screening the nucleus. Thus, there is a gradual increase in the attraction of the nucleus for the peripheral electrons as the nuclear charge increases, and a consequent contraction in atomic radius is observed.

For example, the ionic radii of the +3 cations decrease steadily from a value of 115 pm for La³⁺ to a value of 93 pm for Lu³⁺. The regular decrease in atomic radii with increase in atomic number is known as lanthanoide contraction.