The Actinoids

Full Notes

The actinoids (formerly called actinides) are the 14 elements that follow actinium in the periodic table (atomic numbers 90–103).

They involve the filling of 5f orbitals, which give rise to distinct chemical and physical properties, including variable oxidation states and radioactivity, due to the poor shielding and greater spatial extension of 5f-electrons.

Electronic Configurations

The actinoid series includes 15 elements from Thorium (Th, Z = 90) to Lawrencium (Lr, Z = 103).

The general configuration is [Rn] 5f¹–5f¹⁴ 6d^0–1 7s².

Just like in lanthanoids, the 5f orbitals are progressively filled, but:

• The 5f and 6d orbitals are very close in energy.
• This causes irregular filling, especially in early actinoids.

For ExampleF Th has 5f⁰, Pa has 5f², and U has 5f³ configurations.

Key Point: The filling of 5f orbitals begins in Thorium or Protactinium, and not smoothly as in lanthanoids.

Ionic Sizes

The ionic and atomic radii decrease steadily across the actinoid series. This trend is referred to as the actinoid contraction, similar to lanthanoid contraction.

• Cause: Poor shielding effect of 5f electrons, resulting in increased nuclear attraction.
• Impact:
  • Greater contraction than lanthanoids.
  • Leads to similarities in later actinoids' chemical behaviour.
  • Causes higher charge density and stronger bonding.

Oxidation States

Unlike lanthanoids (mostly +3), actinoids exhibit a wider range of oxidation states, especially the early ones:

Later actinoids (Am to Lr) predominantly show the +3 oxidation state due to greater stability of the 5f shell.

• Why wide variability? Due to small energy difference between 5f, 6d, and 7s subshells.
• Stabilisation: Higher oxidation states are stabilised in the presence of strong ligands like O²⁻ and F⁻.
• Trend: +5, +6, +7 states are more common in early actinoids, but become less stable as we go across.

General Characteristics and Comparison with Lanthanoids

Chemical Reactivity

• Actinoids are highly reactive due to large atomic/ionic size and multiple oxidation states.
• They readily react with:
  • Oxygen to form dioxides or trioxides.
  • Halogens to form trihalides or tetrahalides.
  • Water and acids to liberate H₂ gas.
• Easily form complexes, especially with oxygen and halide ligands.

Radioactivity

• All actinoids are radioactive.
• Thorium and Uranium occur naturally.
• Others (e.g., Np, Pu, Cm) are synthetic and used in nuclear reactors or weapons.

Comparison with Lanthanoids

Why do Actinoids Show More Oxidation States?

Because 5f, 6d, and 7s orbitals are close in energy, electrons can be removed from multiple shells. This allows them to lose variable numbers of electrons, resulting in variable oxidation states.

Example: Uranium Chemistry

• UO₂ → Uranium(IV) oxide
• UO₃ → Uranium(VI) oxide
• UF₆ → Uranium hexafluoride used in nuclear fuel processing

Summary

• Actinoids fill 5f orbitals with general configuration [Rn] 5f¹–5f¹⁴ 6d^0–1 7s².
• Actinoid contraction arises from poor 5f shielding and increases across the series.
• They display a wide range of oxidation states with +3 most common.
• Actinoids are all radioactive, highly reactive and form extensive complexes.
• Compared with lanthanoids they show greater oxidation variability and stronger paramagnetism.