1. Lanthanoids: configuration, oxidation states, colour, complexes and alloys
Lanthanoids: Ce (58) to Lu (71), after La; fill 4f. General configuration ; 5d¹ only in La, Ce, Gd (4f⁷5d¹) and Lu (4f¹⁴5d¹). Ln³⁺: remove 6s², then 5d or one 4f electron.
Oxidation states: +3 for all (lattice/hydration enthalpy repays ionisation). +4: Ce⁴⁺ (f⁰), Tb⁴⁺ (f⁷) — oxidising. +2: Eu²⁺ (f⁷), Yb²⁺ (f¹⁴), Sm²⁺ — reducing.
Colour: f–f transitions; f⁰ and f¹⁴ ions (La³⁺, Lu³⁺) colourless. Unpaired f electrons → paramagnetic. Complexes: few (large ions, 4f buried). Alloys: mischmetall (≈ 95% Ln, 5% Fe, traces S, C, Ca, Al) — flints, Mg alloys for shells and bullets.
Formulas
- Lanthanoid configuration
Key terms
- Lanthanoids
The fourteen elements cerium to lutetium, in which 4f orbitals are being filled.
- Mischmetall
An alloy of about 95% lanthanoid metals and 5% iron with traces of S, C, Ca and Al.
2. Lanthanoid contraction and its consequences
Definition: the steady decrease in atomic and Ln³⁺ ionic radii from La (103 pm) to Lu (86 pm). Cause: 4f electrons shield poorly (diffuse shape), so effective nuclear charge increases steadily.
Consequences: (i) 4d and 5d elements of a group have nearly equal radii (Zr 160, Hf 159 pm; Nb/Ta; Mo/W) and very similar chemistry — Zr and Hf are hard to separate; (ii) basicity of Ln(OH)₃ decreases La → Lu; (iii) lanthanoids are difficult to separate (ion exchange); (iv) 5d metals are very dense.
- Ln³⁺ ionic radius
Key terms
- Lanthanoid contraction
The steady decrease in size of lanthanoid atoms and Ln³⁺ ions with increasing atomic number, due to poor shielding by 4f electrons.
3. Chemical reactivity of lanthanoids
Strongly electropositive ( ≈ −2.3 V); early members similar to calcium; reactivity falls slightly to Lu.
| Reagent | Product |
|---|---|
| O₂ (burn) | Ln₂O₃ (Ce → CeO₂) |
| H₂ (heat) | LnH₂, LnH₃ |
| X₂ (heat) | LnX₃ |
| S (heat) | Ln₂S₃ |
| N₂ (heat) | LnN |
| C (2773 K) | Ln₃C, Ln₂C₃, LnC₂ |
| H₂O | Ln(OH)₃ + H₂ |
| Dilute acid | Ln³⁺ salt + H₂ |
| Reagent | Product |
|---|---|
| O₂ | Ln₂O₃ |
| N₂ | LnN |
| S | Ln₂S₃ |
| C | LnC₂ and others |
Formulas
- Reaction with water
Key terms
- Electropositive
Having a strong tendency to lose electrons and form positive ions.
4. Actinoids and comparison with lanthanoids
Actinoids: Th (90) to Lr (103); fill 5f; . All radioactive; transuranium elements synthetic.
Oxidation states: +3 to +7; maximum +4 (Th), +5 (Pa), +6 (U), +7 (Np, Pu), then falls; later members mainly +3. Reason: 5f, 6d, 7s close in energy; 5f less buried than 4f.
Comparison: actinoids — wider range of states, all radioactive, larger contraction, more complex formation (e.g. uranyl complexes); lanthanoids — mainly +3, only Pm radioactive, smaller contraction, fewer complexes.
- Highest oxidation state of actinoid
| Feature | Lanthanoids | Actinoids |
|---|---|---|
| States | Mainly +3 | +3 to +7 |
| Radioactive | Only Pm | All |
| Complexes | Fewer | More |
Key terms
- Transuranium elements
Elements with atomic number greater than 92, all made artificially.
- Actinoid contraction
Decrease in size across the actinoids, greater than the lanthanoid contraction because 5f electrons shield even more poorly.
Exam tips
Common mistakes
- Giving the cause of the lanthanoid contraction as "increasing number of shells".
- Calling Ce⁴⁺ a reducing agent or Eu²⁺ an oxidising agent.
- Writing lanthanoid oxides as LnO instead of Ln₂O₃.
- Saying all lanthanoids are radioactive (only promethium).
How marks are usually awarded
- Lanthanoid contraction (2–3 marks): definition, cause (poor 4f shielding) and consequences with examples.
- Reactivity (2 marks): correct products with +3 state and balanced equations.
- Comparison (3 marks): three distinct points, preferably in a table.