Lanthanoid contraction — cause and consequences for chemistry of 5d elements

Lanthanoid contraction is the steady decrease in atomic and ionic radii of the 14 lanthanoid elements (Ce to Lu) as we move across the series from left to right. The decrease is small but cumulative — by the end of the series, the size has shrunk significantly.

For example, the ionic radius of $\text{La}^{3+}$ is 106 pm, while $\text{Lu}^{3+}$ is 85 pm — a drop of about 21 pm across 14 elements.

As we move from Ce to Lu, electrons are added to the 4f subshell. The 4f orbitals have a diffuse, complex shape and poor spatial penetration toward the nucleus. This means 4f electrons do a poor job of shielding the outer electrons from the increasing nuclear charge.

With each added proton (nuclear charge increases by +1) and each added 4f electron (which barely shields), the effective nuclear charge felt by the outer electrons increases. This pulls the electron cloud inward, shrinking the atomic radius.

The cumulative effect of lanthanoid contraction profoundly affects the 5d series (Hf to Hg):

• Similar radii of 4d and 5d elements: Zr and Hf have nearly identical atomic radii (160 pm vs 159 pm). Without lanthanoid contraction, Hf would be much larger.
• Separation difficulty: Zr-Hf and Nb-Ta pairs are extremely difficult to separate chemically because their similar sizes give them almost identical chemical properties.
• Higher densities and melting points: 5d elements are denser and have higher melting points than expected because of the smaller-than-predicted atomic size.
• Higher ionisation energies: The 5d elements have unexpectedly high ionisation energies compared to what periodic trends alone would predict.