What is Lanthanide Contraction? Causes and Consequences

Normally, as you move across a period and electrons are added, the size of the atom shrinks slightly due to increased nuclear charge. However, the lanthanide contraction is more pronounced due to the shape of the f-orbitals.

As atomic number increases in the lanthanide series, the new electrons enter the deeply buried 4f subshell. The f-orbitals are highly diffused in shape. Because of this diffused shape, 4f electrons offer very poor shielding (or screening) for the outermost valence electrons against the pull of the positively charged nucleus. As the nuclear charge increases, the nucleus pulls the outer electrons inward much more strongly than expected, causing the atomic radius to contract steadily.

The most famous consequence of the lanthanide contraction is seen in the transition metals that come after the lanthanides. Normally, atomic size increases as you go down a group (from 3d to 4d to 5d). However, because of the massive contraction that happens in the f-block, the elements in the 5d transition series (like Hafnium) end up being almost exactly the same size as the elements directly above them in the 4d series (like Zirconium). Because Zr and Hf have identical sizes, their chemical properties are nearly identical, making them incredibly difficult to separate.

Because the atomic size shrinks and the effective nuclear charge is so strong, it becomes much harder to remove an electron. Consequently, the ionization energy and electronegativity of elements following the lanthanides are unexpectedly high.

As the size of the lanthanide ions (Ln³⁺) decreases from La³⁺ to Lu³⁺, the covalent character of their hydroxide bonds (Ln-OH) increases. Therefore, the basic strength of the lanthanide hydroxides steadily decreases from Lanthanum hydroxide to Lutetium hydroxide.