Chemistry - f-block Question with Solution | TestHub

Solution: As the atomic number increases across the lanthanide series, the atomic and ionic radii generally decrease due to poor shielding by 4f electrons, a phenomenon known as lanthanide contraction. However, Europium (Eu) has an exceptionally large atomic radius due to its stable half-filled f-orbital configuration, which leads to weak metallic bonding.

The general trend is Ce > Gd > Lu.

Due to its 4f⁷ configuration, Eu has an unusually large size.

The correct order is Eu > Ce > Gd > Lu.