Q1(j). Anomalous lanthanide radii

Answer in brief

Europium and ytterbium have anomalously large metallic radii because their metals favour divalent character.

Key points

• Eu has the especially stable 4f⁷ configuration and Yb the 4f¹⁴ configuration.
• Most neighbouring lanthanide metals have approximately trivalent metallic bonding. Eu and Yb therefore have weaker effective metallic cohesion and larger interatomic separations.
• The anomaly concerns atomic/metallic radii. It should not be confused with the generally smooth contraction of the trivalent ionic radii across the series.