The lanthanoids and actinoids, and uses of d- and f-block elements

The lanthanoids: configuration and oxidation states

The lanthanoids are the 14 elements from cerium (58) to lutetium (71), with lanthanum (57) studied along with them. Their atoms are [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s², and their M³⁺ ions are [Xe] 4fⁿ. The +3 state is the most common for all of them. Some show +2 or +4 when this gives an empty (f⁰), half-filled (f⁷) or full (f¹⁴) 4f set. Ce⁴⁺ (4f⁰) is a strong oxidising agent (E° Ce⁴⁺/Ce³⁺ = +1.74 V), because it tends to return to +3. Tb⁴⁺ (4f⁷) is also an oxidant. Eu²⁺ (4f⁷) and Yb²⁺ (4f¹⁴) are strong reducing agents, because they tend to change to +3.

Lanthanoid contraction: cause and consequences

1. 1
   From La to Lu, the nuclear charge rises by one at each step, and one electron enters the 4f subshell.
2. 2
   4f orbitals are diffuse, so one 4f electron shields another very imperfectly from the nucleus.
3. 3★
   The effective nuclear charge felt by the outer electrons rises, so atomic and ionic radii decrease steadily: this is the lanthanoid contraction.
4. 4
   Consequence 1: 5d metals are almost the same size as the 4d metals above them (Zr 160 pm, Hf 159 pm), so Zr and Hf occur together and are hard to separate.
5. 5
   Consequence 2: the lanthanoids are so alike in size and properties that they are difficult to separate from one another.
6. 6
   Consequence 3: basic strength of the hydroxides decreases from La(OH)₃ to Lu(OH)₃, as the smaller ion holds OH⁻ more covalently.

★ key step

General characteristics of the lanthanoids

• Silvery white, soft metals that tarnish quickly in air. Hardness rises with atomic number; samarium is steel-hard.
• Good conductors of heat and electricity; high melting points (about 1000 to 1200 K, samarium 1623 K).
• Many Ln³⁺ ions are coloured in the solid and in solution, because of f-f transitions. La³⁺ (4f⁰) and Lu³⁺ (4f¹⁴) are colourless.
• Ions are paramagnetic except the f⁰ (La³⁺, Ce⁴⁺) and f¹⁴ (Yb²⁺, Lu³⁺) types.
• First ionisation enthalpies are fairly low (about 600 kJ mol⁻¹), close to that of calcium. E°(Ln³⁺/Ln) is about −2.2 to −2.4 V (Eu about −2.0 V). The early members are reactive like calcium; later ones behave more like aluminium.
• Reactions: with dilute acids they give H₂; with water, Ln(OH)₃ and H₂; burn in O₂ to give Ln₂O₃; with halogens, halides; heated with N₂, LnN; with sulphur, Ln₂S₃; heated with carbon, carbides Ln₃C, Ln₂C₃ and LnC₂ (LnC₂ at about 2773 K).

The actinoids

The actinoids are the 14 elements from thorium (90) to lawrencium (103), studied with actinium (89). All are radioactive; the later ones have very short half-lives and are made only in nanogram amounts. Their atoms are [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s². The 5f, 6d and 7s levels have similar energies, so actinoids show more oxidation states than lanthanoids. The highest state rises to +4 in Th, +5 in Pa, +6 in U and +7 in Np (Pu also reaches +7), then falls in later elements; +3 is common. Sizes fall from Th to Lr, and this actinoid contraction is larger from one element to the next than the lanthanoid contraction, because 5f electrons shield even more poorly. The metals are silvery but have varied structures. They are highly reactive, especially when finely divided: hydrochloric acid attacks all of them, but most are only slightly affected by nitric acid because a protective oxide layer forms. Early actinoids have lower ionisation enthalpies than early lanthanoids: 5f electrons penetrate less into the inner core, so they are more effectively shielded and are easily used for bonding. Their magnetic properties are more complex than those of the lanthanoids.

Lanthanoids vs actinoids

Some applications of d- and f-block elements

• Iron and steels are the most important construction materials.
• TiO₂ is used in the pigment (white paint) industry; MnO₂ in dry cells; Zn and Ni/Cd in batteries.
• Group 11 metals (copper, silver and gold) are the coinage metals; AgBr is used in photography.
• Catalysts: V₂O₅ in the Contact process; TiCl₄ with Al(CH₃)₃ (Ziegler catalyst) to make polythene; iron in the Haber process; nickel to hydrogenate fats; PdCl₂ in the Wacker process (oxidation of ethene to ethanal; the textbook misprints this as ethyne); nickel complexes to polymerise alkynes.
• Mischmetall (about 95% lanthanoid metal, 5% iron) is used with magnesium in bullets, shells and lighter flints.
• Mixed lanthanoid oxides are catalysts in petroleum cracking; some lanthanoid oxides are phosphors in television screens. Uranium and plutonium are nuclear fuels.