Many Ln³⁺ ions have partly filled 4f subshells, giving paramagnetism and characteristic weak colours from f–f transitions. The 4f orbitals lie inside filled outer 5s and 5p shells, so their spectroscopic energies are less affected by ligands than the more exposed d orbitals of transition complexes. “Less sensitive” does not mean absolutely invariant under every environment.

The associated f⁰ ions La³⁺ and Ce⁴⁺ and f¹⁴ ions Lu³⁺ and Yb²⁺ have no unpaired f electrons and are diamagnetic in the elementary count. They lack ordinary f–f transitions. However, Ce(IV) compounds can still be yellow/orange through charge-transfer absorption, just as d⁰ permanganate is coloured. Thus f⁰ means absence of f–f colour, not that every actual salt solution must be transparent.

For a simple unpaired-count model, f has seven orbitals: f¹ through f⁷ have one through seven unpaired, then f⁸ through f¹⁴ have six through zero. Gd³⁺ and Eu²⁺ are f⁷. In actual lanthanoid moments both spin and orbital angular momentum usually matter, so the first-row d-ion spin-only formula is not an exact general f-block law. Keep model-based electron counts separate from experimentally measured moments, and specify oxidation state before classifying colour or magnetism.

Seven f orbitals; f⁷ has seven unpaired in the basic model; f⁰/f¹⁴ have zero

Worked example

Gd³⁺ is f⁷ and paramagnetic; Lu³⁺ is f¹⁴ and diamagnetic. Yellow Ce(IV) can be a charge-transfer exception without acquiring an f electron.

NEET Method

Count f electrons, then distinguish ordinary f–f transitions, charge transfer and orbital magnetic contributions.

NCERT Trap

A Ce⁴⁺ f⁰ configuration does not prohibit coloured Ce(IV) compounds; avoid applying spin-only d-ion moments universally to lanthanoids.

d- and f-Block Elements

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