The Lanthanoids

Full Notes

The lanthanoids (previously called lanthanides) are the 14 elements that follow lanthanum in the periodic table (atomic numbers 58–71).

They involve the filling of 4f orbitals, which results in unique properties due to the poor shielding effect of f-electrons.

Electronic Configurations

Lanthanoids are characterized by the filling of the 4f subshell, following the configuration: [Xe] 4f¹–4f¹⁴ 5d^0–1 6s²

• The 4f orbitals are gradually filled from Ce (Z = 58) to Lu (Z = 71).
• The electrons enter the 4f orbital, but irregularities exist due to energy proximity with 5d orbitals.
• For example, Gd (Z = 64) shows [Xe] 4f⁷ 5d¹ 6s² instead of [Xe] 4f⁸ 6s² to gain extra stability from a half-filled 4f⁷ configuration.

Atomic and Ionic Sizes

There is a gradual decrease in atomic and ionic radii with increasing atomic number across the lanthanoid series – a trend called the lanthanoid contraction.

Why lanthanoid contraction happens:

• 4f electrons do not shield the nuclear charge effectively.
• Increased effective nuclear charge pulls electrons closer, decreasing size.

Consequences:

• Causes similar sizes among the 2nd and 3rd transition series.
• Affects the chemical separation of lanthanoids.
• Leads to slight differences in chemical properties, even though they have similar outer electronic configurations.

Oxidation States

• The +3 oxidation state is the most stable and common across all lanthanoids.
• A few elements show +2 and +4 states as well due to extra stability of empty, half-filled or fully filled f-orbitals:
  • Ce⁴⁺ ([Xe] 4f⁰): stable due to empty f-subshell.
  • Eu²⁺ ([Xe] 4f⁷): stable due to half-filled 4f.
  • Yb²⁺ ([Xe] 4f¹⁴): stable due to fully filled 4f.
  • Tb⁴⁺ ([Xe] 4f⁷): half-filled stability.
• Redox Nature: The +2/+4 states are often strong oxidising or reducing agents and are readily converted to +3 state in aqueous solution.

Physical Properties of Lanthanoids

• All lanthanoids are silvery white, soft metals.
• They tarnish quickly in air due to oxidation.
• Hardness increases with atomic number; Samarium is steel-hard.
• Melting points range from 1000 to 1200 K, with Samarium at 1623 K.
• Have typical metallic structure and are good conductors of heat and electricity.
• Density and other properties vary smoothly with atomic number except for Europium (Eu), Ytterbium (Yb), Samarium (Sm), and Thulium (Tm).

Colour and Magnetic Behaviour

• Many trivalent lanthanoid ions are coloured in both solid and aqueous state, due to f-electrons.
• La³⁺ and Lu³⁺ are colourless; they have f⁰ and f¹⁴ configurations respectively.
• Most other ions show narrow absorption bands due to transitions within f-orbitals.
• Paramagnetism:
  • La³⁺ (f⁰) and Ce⁴⁺ (f⁰): Not paramagnetic.
  • Yb²⁺ (f¹⁴) and Lu³⁺ (f¹⁴): Not paramagnetic.
  • All others: Paramagnetic due to unpaired f-electrons.

Ionisation Enthalpies

• 1st ionisation enthalpy ≈ 600 kJ/mol
• 2nd ionisation enthalpy ≈ 1200 kJ/mol → Comparable to calcium.
• The 3rd ionisation enthalpy is crucial for forming Ln³⁺ ions and reflects stability from empty, half-filled, and fully filled f-orbitals.
• Exceptionally low 3rd ionisation enthalpies for La, Gd, and Lu are due to stable electronic configurations.

Chemical Reactivity

• Generally reactive, especially early lanthanoids; similar to calcium in reactivity.
• Reduces to metal as: Ln³⁺(aq) + 3e⁻ → Ln(s)
• Standard electrode potentials (E°) range from −2.2 V to −2.4 V, except Eu (−2.0 V).

Reactions of Lanthanoids

• Burn in O₂ forms Ln₂O₃
• With halogens forms LnX₃
• With acids forms Liberate H₂ gas
• With water forms Ln(OH)₃ + H₂
• With nitrogen forms LnN
• With carbon at 2773 K forms LnC₂
• With sulphur forms Ln₂S₃

Important Compounds

• Oxides (M₂O₃) and hydroxides (M(OH)₃) are:
  • Definite compounds (not just hydrated oxides)
  • Basic in nature, like alkaline earth metal hydroxides

Uses

• Mischmetall: An alloy (~95% lanthanoid metals, ~5% Fe, traces of S, C, Ca, Al)
  • Used in Mg-based alloys (e.g., for bullets, shell, lighter flint)
• Lanthanide mixed oxides:
  • Catalysts in petroleum cracking
  • Used in television phosphors and fluorescent surfaces

Summary

• Lanthanoids fill the 4f subshell and show lanthanoid contraction.
• They are soft reactive metals with predominant +3 oxidation state.
• Trivalent ions are often coloured and paramagnetic due to f-electrons.
• Key applications include catalysts and mischmetall alloy production.