Actinoid

The group of fifteen elements from Actinium (Ac, Z = 89) to Lawrencium (Lr, Z = 103) is collectively referred to as actinides, actinoids, or actinons. These elements follow actinium (Ac, Z = 89) in the periodic table, which is why they bear this name. Classified as inner transition elements, actinides are positioned between actinium and rutherfordium (Rf, Z = 104), corresponding to the fourth transition series. Notably, all actinides are radioactive metals and play a crucial role in nuclear chemistry. 

Transuranium Elements 

Transuranium elements are actinides that come after uranium in the periodic table. These include elements with atomic numbers from 93 (neptunium) to 103 (lawrencium).

 Electronic Configuration: 

The electron arrangement of actinides does not follow a straightforward pattern like the lanthanides. After lanthanum (La), the 4f orbitals become lower in energy than the 5d orbitals. Similarly, one might expect the 5f orbitals to be lower than the 6d orbitals after actinium (Ac), but this is not immediately the case.

 For the first few actinides—thorium (Th), protactinium (Pa), uranium (U), and neptunium (Np)—the energy levels of 5f and 6d orbitals are quite close. Therefore, electrons may fill either orbital or both. However, starting from plutonium (Pu), the 5f orbital becomes significantly more stable (lower in energy), and electrons start filling it more consistently. As a result, the properties of the elements from Pu to Lr become very similar. 

The general valence shell electronic configuration of the actinide elements may be written as: 5f 0-146d 0-27s2.

 Occurrence:

 Actinium (Ac, Z = 89), although used to name the series, is not very abundant. It was discovered in 1902 by Friedrich Otto Giesel and is found in uranium ores. It is about 150 times more radioactive than radium. Naturally occurring actinides include thorium (Th), protactinium (Pa), uranium (U), and actinium (Ac), which are extracted from minerals, especially in countries like Canada, the USA, South Africa, and Namibia. The rest of the actinides are synthetic and produced in laboratories.

 Oxidation States: 

All actinides exhibit the +3 oxidation state, which becomes more stable with increasing atomic number.

 The +4 state is shown from Th to Bk.

 The +5 and +6 states are observed from Th to Am and U to Am, respectively.

+7 oxidation state is rare and found only in Np and Pu, where Np(VII) acts as a strong oxidising agent. The +2 state is observed in limited compounds like ThBr₂ and ThI₂.

 They form two main types of cations: M³⁺ and M⁴⁺, and oxo-cations such as MO₂⁺ (M⁵⁺) and MO₂²⁺ (M⁶⁺), which are stable in acidic solutions. For example: UO₂⁺, PuO₂⁺, UO₂²⁺, PuO₂²⁺.

Np³⁺ and Pu³⁺ are readily oxidised to Np⁴⁺ and Pu⁴⁺ in solution by air.

 Lower oxidation states are ionic; higher ones show covalent character.

 Disproportionation Reaction Example:

2UO₂⁺ (+v) + 4H⁺ → U⁴⁺ (+iv) + UO₂²⁺ (+vi) + 2H₂O 

 Note: +5 oxidation state ions are unstable in solution, but they exist as MO₂²⁺ between pH 2–4 and undergo disproportionation.

Atomic and Ionic Radii: 

Atomic radii decrease from Th to Np, then increase slightly up to Bk.

 Ionic radii of M³⁺ and M⁴⁺ ions consistently decrease across the series.

 This gradual decrease is termed actinide contraction, similar to lanthanide contraction.

 Cause: Increasing nuclear charge and poor shielding by 5f electrons.

Magnetic Properties: 

Actinides, like lanthanides, exhibit paramagnetism due to unpaired electrons. However, ions like Th⁴⁺ and U⁶⁺ are diamagnetic, as they have no unpaired electrons and possess electron configurations similar to that of radon (Rn, Z = 86).

 A key difference between actinides and lanthanides is the strong spin-orbit coupling in actinides (2000–4000 cm⁻¹), particularly in lighter actinides where the 5f and 6d orbitals are close in energy. This leads to complex magnetic behaviour, and each compound must be evaluated individually.

 The standard equation for magnetic moment, μ = g√J(J + 1), is less applicable to actinides, as their magnetic moments are:

 Lower than expected, strongly temperature-dependent compared to those of lanthanides.

Electronic Spectra (UV-Vis) of Actinides: 

f–f Transitions in Actinides: 

 f-f transitions in actinides are formally Laporte-forbidden, meaning they are typically weak. However, due to the distortion caused by crystal fields, the selection rules are partially relaxed. Compared to lanthanides, actinides experience stronger crystal field interactions, which results in:

 Narrow and complex spectral bands, Higher intensity of these bands

Transitions appearing in the UV-visible region, often giving rise to distinct colors in aqueous solutions of simple actinide compounds.

5f-6d Transitions: 

These transitions are both Laporte- and spin-allowed, resulting in:

Broader and more intense bands

Bands are typically in the UV region

Minimal contribution to the visible color of ions

Compounds of Actinides

Actinides are highly reactive and electropositive, resembling lanthanides but forming more complexes.

(i) Oxides:

React with air or O₂ to form various oxides

Uranium oxides: UO, UO₂, U₃O₈, UO₃

U + O₂ → U₃O₈ (on heating)

UO₃ → UO₂ (on heating)

Other oxides:

Np: NpO, NpO₂, Np₃O₈

Pu: PuO, Pu₂O₃, Pu₂O₇, PuO₂

Am: AmO, AmO₂

(ii) Hydrides, Nitrides, and Carbides:

U and Pu form hydrides: UH₃, PuH₃

U + H₂ → UH₃ (reaction faster at 250°C)

UH₃ is reactive and hydrolyzed by water:

2UH₃ + 4H₂O → 2UO₂ + 7H₂

Reacts with Cl₂, HF, HCl to form UF₄, UCl₄, etc.

On reacting with NH₃:

Forms nitrides like UN, U₂N₃, UN₂, PuN

Also forms carbides: UC, PuC

All MX-type compounds (X = O, C, N) have rock salt structure

Halides

Actinides react with halogens or hydrogen halides to form halides, particularly U and Np.

Common halides: UF₆, UF₅, UF₄, UF₃, UCl₄, UCl₃, UBr₄, UBr₃

Trihalides (MX₃) are isomorphous

Higher halides like tetra-, penta-, and hexa-halides are also common

Example reactions:

U + 3Cl₂ → UCl₆

U + F₂ → UF₆

U + 2F₂ → UF₄

Neptunium, plutonium, and americium show similar reactivity and form analogous halides.