Neptunium, plutonium, and americium

Key points: Oxidation states higher than +3 become increasing less accessible between Np and Am, although all these actinoids can form AnO₂ⁿ⁺ species in aqueous solution. The three elements Np, Pu, and Am form compounds containing similar species, although there are significant differences in the stabilities of the main oxidation states. The Frost diagrams in Fig. 23.9 summarize their behaviour. Neptunium dissolves in dilute acids to produce Np³, which is readily oxidized by air to produce Np⁴⁺. Increasingly strong oxidizing agents produce NpO2 (Np(V)) and NpO₂⁺² (Np (VI)). The four oxidation states of plutonium, Pu (III), Pu (IV), Pu(V), and Pu(VI), are separated from each other by less than 1 V and solutions of Pu often contain a mixture of the species Pu³⁺, Pu⁴⁺, and PuO₂⁺² (PuO⁺² has a tendency to disproportionate to Pu⁴⁺ and PuO₂²⁺). The ion Am³⁺ is the most stable species in solution, reflecting the tendency for the An (III) to dominate actinoid chemistry for the high atomic number elements. Under strongly oxidizing conditions AmO₂ and AmO₂⁺² can be formed; Am (IV) disproportionates in acidic solutions. The An (IV) oxides NpO₂, PuO₂, and AmO₂, which are formed by heating the elements or their salts in air, all adopt the fluorite structure. Lower oxides include Np₃O₈, Pu₂O₃, and Am₂O₃. The trichlorides, AnCl₃, can be obtained by direct reaction of the elements at 450°C and have structures analogous to that of LnCl₃ with a nine-coordinate An atom. Tetrafluorides are known for all three actinoids though only Np and Pu form tetrachlo rides, further demonstrating the difficulty in raising americium to Am (IV). Both Np and Pu form hexafluorides which, like UF₆, are volatile solids. All three metals have species analogous to the uranyl ion, forming NpO₂⁺², PuO₂⁺², and AmO₂⁺², which can be extracted from aqueous solution by tributylphosphate as AnO₂ (NO₃)₂ (OP(OBu)₃)₂. The tetrahalides are Lewis acids and form adducts with electron pair donors such as DMSO, as in AnCl₄ (Me₂SO)₇. Neptunium forms a number of organo metallic compounds that are analogues of those of uranium, such as Np (Cp)₄. Although these elements are highly toxic due to their radioactivity, this property is put to good use in many smoke detectors, which contain a minute amount (0.2 μg) of 241Am. This isotope is an α-emitter with a half-life of 432 years and the α-particles enter a space between two electrodes where they ionize the air, so permitting a small current to flow between the electrodes. However, when smoke particles enter the space between the electrodes they absorb the α-particles (and any ions formed by the α-particles adhere to the smoke particle surfaces) and the resulting reduction in current is detected, setting off the alarm. The isotope ²⁴¹Am is used in this application as it is a strong source of α-particles; however these do not escape from the smoke detector due to their short path-lengths in solids and do not present a hazard.

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مواضيع ذات صلة

Heating

Expansion against constant pressure

The general expression for work

Work, heat, and energy

Mixtures of gases

Future directions

The contributions of individual elements

Imaging agents

Anti-arthritis drugs

Cancer treatment

Chelation therapy

Biomineralization