Zr and Hf possess almost identical radii because of

Inner transition Elements

The lanthanoid contraction is a feature of f block elements in which overall decrease in atomic radii from lanthanum to lutetium is observed. The decrease in atomic radii is not quite regular as it is regular in M³⁺ ions. This is because of imperfect shielding of one electron by another in the same subshell. The shielding of one 4f electron by another is less than one d electron by another with the increase in nuclear charge along the series. In the lanthanoid La(II) and Ln(III) are predominant species. Occasionally +2 and +4 ions in solution or in solid compounds are obtained. The formation of Ce^IV is favored by its noble gas configuration but it acts like a strong oxidising agent reverting to the common +3 state. The E⁰ value for Ce^IV/Ce^III is +1.74 V which refer that it can oxidise water. The Pr, Nd, Tb and Dy show +4 state in oxides only. Eu²⁺ is a strong reducing agent changing to the common +3 state. Actinoids include 14 elements from Th to Lr. There is gradual decrease in size of atoms or M³⁺ ions in series and referred as actinoid contraction. The contraction is greater from element to element in this series resulting from poor shielding by 5f electrons.