Neptunium, plutonium, and americium
المؤلف:
Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong
المصدر:
Shriver and Atkins Inorganic Chemistry ,5th E
الجزء والصفحة:
ص596-597
2025-10-07
242
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.
الاكثر قراءة في مواضيع عامة في الكيمياء العضوية
اخر الاخبار
اخبار العتبة العباسية المقدسة
