Concept 05 / 06The d and f Block Elements
Lanthanoids: Electronic Configuration, Oxidation States and the Lanthanoid Contraction
The inner transition elements, electronic configurations of lanthanoids, the common +3 oxidation state and exceptional +2 and +4 states (Eu, Yb, Ce, Tb), the lanthanoid contraction and its causes and consequences, general physical and chemical properties, and uses including mischmetall.
The f-block elements (inner transition elements) fill the (lanthanoids, Ce to Lu, after La) or (actinoids, Th to Lr, after Ac) orbitals. Lanthanum is usually included with the lanthanoids for comparison.
Electronic configuration. . The and levels are close in energy: La , Ce , Pr , and so on; Gd (half-filled f) and Lu keep one 5d electron. The ions are ( for to for ).
Oxidation states. is the characteristic and most stable state for all. Some show or where this gives an empty, half-filled or filled f subshell:
(): a strong oxidant (), used in analysis, though kinetically slow to react with water.
(): an oxidant.
() and (): strong reductants, readily oxidised to .
() also exists, as a reductant.
Lanthanoid contraction (see figure). A steady decrease in atomic and ionic radii from La to Lu. Each added electron enters the subshell, which shields the outer electrons poorly (f orbitals are diffuse), so the effective nuclear charge increases and the whole electron cloud contracts. The total contraction over the series ( to ) is significant.
Consequences:
the 4d and 5d transition elements of the same group have nearly equal radii (Zr and Hf, Nb and Ta, Mo and W), so they have very similar properties and occur together;
the lanthanoids themselves are very similar and hard to separate (ion exchange and solvent extraction methods exploit tiny differences);
basicity of the hydroxides decreases from to , as the smaller ions polarise more (covalent character increases);
the 5d elements have high densities and ionisation enthalpies.
Properties. Silvery-white, soft metals that tarnish in air; hardness increases with atomic number (Sm is steel-hard); high melting points ( to , Sm ); good conductors. Many ions are coloured (f-f transitions), except (, ) and (, ); many are paramagnetic.
Chemistry: early members are nearly as reactive as calcium. They burn in halogens to give ; with oxygen, ; with sulfur, ; with nitrogen on heating, LnN; with carbon at , carbides (, , ); with water, and ; with dilute acids, . The oxides and hydroxides are basic, like those of alkaline earth metals.
Uses: mischmetall (about lanthanoid metals, iron and traces of S, C, Ca, Al) in magnesium-based alloys for bullets, shells and lighter flints; mixed lanthanoid oxides as petroleum-cracking catalysts; phosphors in television screens and fluorescent lamps; alloy steels.
Key Takeaways:
- ; common state .
- and oxidants; , reductants (empty, half-filled or filled 4f).
- Lanthanoid contraction: poor shielding by 4f; Zr ≈ Hf; basicity of decreases.
- Mischmetall: lanthanoids + Fe, for flints and alloys.
- Pitfall: the contraction per element is small; it is the cumulative effect across elements that matters.
Brain-Teaser 1: Why is a good oxidising agent and a good reducing agent in aqueous solution?
Solution: Both tend towards the common state. (, noble gas core) accepts an electron to become (). () loses an electron to become ; although is half-filled, the greater stability of the state in water (high hydration enthalpy of ) makes a reductant.
Brain-Teaser 2: Arrange , and in order of basic strength and explain.
Solution: . Because of the lanthanoid contraction, size decreases, so the polarising power of the cation increases and the M-OH bond becomes more covalent, making it harder to release .
Brain-Teaser 3: Calculate the spin-only magnetic moment of and state whether is coloured.
Solution: is with unpaired electrons: BM. is (all paired, no f-f transitions): colourless and diamagnetic.
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