Actinoids

The general ground-state electronic configuration of actinoids is [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s². As in the lanthanoids, there are exceptions: thorium is [Rn] 6d² 7s² (no 5f electrons in the ground state), and several others such as Pa ([Rn] 5f² 6d¹ 7s²) show mixed 5f/6d occupancy. The proximity of the 5f, 6d, and 7s orbital energies means that small perturbations (e.g., oxidation, ligand fields) can shift electron populations between subshells, leading to more complex chemistry than seen in lanthanoids.

Every actinoid isotope is radioactive. Radioactive decay occurs because nuclei with too many protons relative to neutrons are energetically unstable. The three principal decay modes are: (1) Alpha decay — emission of a ²He nucleus (α particle), reducing atomic number by 2 and mass number by 4; e.g., ²³⁸U → ²³⁴Th + ⁴He. (2) Beta decay — emission of an electron (β⁻) when a neutron converts to a proton, increasing atomic number by 1; e.g., ²³⁴Th → ²³⁴Pa + e⁻ + ν̄ₑ. (3) Gamma emission — high-energy photons released alongside α or β decay. Half-lives range enormously: ²³⁸U has t₁/₂ ≈ 4.47 × 10⁹ years (comparable to Earth's age), while ²⁵⁷Fm has t₁/₂ ≈ 100.5 days, and some californium isotopes have half-lives of seconds. This radioactivity makes handling actinoids hazardous and requires shielded laboratories.

The early actinoids (Th through Am) display the widest variety of oxidation states among all elements. Uranium, for example, forms compounds in +3 (UCl₃), +4 (UO₂, UF₄), +5 (UF₅), and +6 (UO₃, UF₆) states. The dioxo cations (uranyl ion UO₂²⁺ and neptunyl ion NpO₂⁺) are unique to actinoids and reflect 5f covalent bonding with oxygen. As the series progresses beyond curium (Cm), the 5f electrons become more stabilized (analogous to lanthanoid contraction effects), and the +3 state becomes dominant — more like the lanthanoid pattern. The table below summarizes common oxidation states: Th: +4; Pa: +5; U: +3,+4,+5,+6; Np: +3,+4,+5,+6,+7; Pu: +3,+4,+5,+6; Am: +2,+3,+4,+5,+6; Cm–Lr: mainly +3.

Similar to the lanthanoid contraction, moving across the actinoid series from Ac to Lr, successive addition of 5f electrons increases nuclear charge while 5f electrons provide poor shielding. This causes a gradual decrease in ionic radii. However, because 5f orbitals are more spatially extended than 4f orbitals, the actinoid contraction is less regular and somewhat less pronounced than the lanthanoid contraction. Relativistic effects are also significant: the high nuclear charge accelerates inner electrons to relativistic speeds, contracting and stabilizing s and p orbitals while expanding and destabilizing d and f orbitals. This modifies bond lengths, ionization energies, and chemical behaviour in ways not predicted by classical atomic theory.

Both series fill f-subshells and show similar 'inner transition' behaviour, but key differences exist: (i) Lanthanoids fill 4f (shielded, contracted); actinoids fill 5f (more diffuse, available for bonding). (ii) Lanthanoids are almost exclusively +3; actinoids (early members) show +2 to +7. (iii) Lanthanoids are non-radioactive or very mildly so (e.g., ¹⁴⁷Sm); all actinoids are radioactive. (iv) Lanthanoids form predominantly ionic compounds; early actinoids form significant covalent character (e.g., U–O bonds in UO₂²⁺). (v) Relativistic effects are much larger in actinoids due to higher Z. (vi) Lanthanoids are found in nature in appreciable amounts (in minerals like monazite); most actinoids beyond U are synthetic.

• ^{238}_{92}\text{U} — Uranium-238 (—): Parent radioactive nuclide; the most abundant natural isotope of uranium, with a half-life of 4.47 × 10⁹ years.
• ^{234}_{90}\text{Th} — Thorium-234 (—): Daughter nuclide produced after α emission; atomic number reduced by 2, mass number reduced by 4.
• ^{4}_{2}\text{He} — Alpha particle (—): Helium-4 nucleus (2 protons + 2 neutrons) emitted during alpha decay; has low penetrating power but high ionising ability.

Problem. Uranium hexafluoride (UF₆) is used in uranium enrichment for nuclear fuel. (a) Identify the oxidation state of uranium in UF₆. (b) Write the balanced equation for the formation of UF₆ from solid uranium metal and fluorine gas. (c) Explain why uranium can achieve this oxidation state while the corresponding lanthanoid (neodymium, Nd, which is in an analogous position in the 4f series) typically does not exceed +3.

Answer. U is +6 in UF₆; formation equation: U(s) + 3F₂(g) → UF₆(g); U achieves +6 due to accessible 5f electrons for bonding, unlike Nd whose 4f electrons are too tightly bound.

• Lanthanoids — Lanthanoids are the f-block row immediately above actinoids; comparing their electronic configurations, oxidation states, and ionic radii is essential to understand the distinct chemical behaviour of actinoids.
• Oxidation States of Transition Metals — Understanding how d-block elements display variable oxidation states provides the conceptual framework for appreciating why early actinoids show an even wider range due to accessible 5f, 6d, and 7s electrons.
• Radioactivity and Nuclear Decay — All actinoids are radioactive; understanding alpha decay, beta decay, gamma emission, and half-life calculations is prerequisite knowledge for nuclear chemistry and actinoid applications.
• Introduction to f-Block Elements — The f-block includes both lanthanoids and actinoids; this concept provides the broader context of inner-transition element electronic structure and periodic table placement.