Actinium stability

Actinium-225 decays by successive emission of three u particles, (a) Write the nuclear equations for the three decay processes, (b) Compare the neutron-to-proton ratio of the final daughter product with that of actinium-225. Which is closer to the band of stability ... 

A radioactive element is an element that disintegrates spontaneously with the emission of various rays and particles. Most commonly, the term denotes radioactive elements such as radium, radon (emanation), thorium, promethium, uranium, which occupy a definite place in the periodic table because of their atomic number. The term radioactive element is also applied to the various other nuclear species, (which arc produced by the disintegration of radium, uranium, etc.) including (he members of the uranium, actinium, thorium, and neptunium families of radioactive elements, which differ markedly in their stability, and are isotopes of elements from thallium (atomic number 81) to uranium (atomic number... 

As early as 1923 N. Bohr suggested that there might exist a group of IS elements at the end of the Periodic Table Uiat would be analogous in their properties to the IS lanthanide ("rare earth") elemrats. This idea, combined with the increasing stability of the -t-3 oxidation state for the transuranium elem ts as the atomic number increases from Z = 93 to 96, led Seaborg to the conclusion that these new elemrats constituted a second rare earth series whose initial member was actinium. As the atomic number increases from 90, electrons are added in the Sf subshell similar to the occupation of the 4f subshell in the lanthanides, see Table 16.1. This series would be terminated with element 103 since this would correspond to the addition of 14 electrons for a completed Sf subshell. 

The chemical properties span a range similar to the representative elements in the first few rows of the periodic table. Francium and radium are certainly characteristic of alkah and alkaline earth elements. Both Fr and Ra have only one oxidation state in chemical comhina-tions and have little tendency to form complexes. Thallium in the 1+ oxidation state has alkah-like properties, but it does form complexes and has extensive chemistry in its 3+ state. Similarly, lead can have alkaline earth characteristics, hut differs from Ra in forming complexes and having a second, 4+, oxidation state. Bismuth and actinium form 3+ ions in solution and are similar to the lanthanides and heavy (Z > 94) actinides. Thorium also has a relatively simple chemistry, with similarities to zirconium and hafiuum. Protactinium is famous for difficult solution chemistry it tends to hydrolyze and deposit on surfaces unless stabilized (e.g., by > 6 M sulfuric acid). The chemistry of uranium as the uranyl ion is fairly simple, hut... 

The multiplicity of oxidation states of the light actinides can be utilized to accomplish very efficient separation of these elements from the lanthanides. Except for actinium (only trivalent), the actinide ions to plutonium either exist predominantly in higher oxidation states [Th(IV), Pa(IV, V)] or can be interconverted with relative ease among any of four oxidation states (III, IV, V, VI). The upper two oxidation states exist in aqueous solutions as the dioxocations AnOj or AnO - The relative strength of complexes formed by the actinide cations in these oxidation states is An(IV) > An(VI) > An(III) > An(V), which order also applies to the separation reactions involving these cations. The dominant oxidation states for the light actinides are Ac(III), Th(IV),Pa(IV or V),U(IV or VI), Np(IV or V). For plutonium, the redox potentials indicate nearly equal stability for all four oxidation states in acidic solutions. The tri-, tetra-, and hexavalent oxidation states are most important in separations. 

The kind of analysis outlined above can yield accurate assessments of the extent to which f electrons participate in the chemical bonding in the lighter actinides, but they are dependent upon accurate experimental data for actinium and the transplutonium elements. Both thermochemical and optical spectroscopic data are also useful for analysis of the factors determining the oxidation states of the actinide atom in compounds (Johansson 1977a, Brooks et al. 1984). For example, the stability of 450 different halides and oxides of the lanthanides were investigated, and the existence or non-existence of di-, tri- and tetravalent compounds was accounted for very well (Johansson 1977a). 

Actinium is the first element of the actinide group. This element and its lighter homologue La have ndi/iin + 1) D /2 ground states, with n = 5 in La and 6 in Ac. The heavy eka-Ac (element 121) has been predicted [16] to have an 85 8p Pi/2 ground state, due to the sizable relativistic stabilization of the 8pi/2 electron. These three atoms were the subject of an early application of our RFSCC approach [16]. 

All isotopes of actinium are radioactive and exist in aqueous solution only in the trivalent state. All of the isotopes of actinium are relatively short lived, with the longest half-life being 21.6 years for Ac. Consequently, it is difficult to measure the stability of its aqueous species or the solubility of its phases. 

There are no data reported in the literature that give reliable stability or solubility constants for the hydrolytic species of uranium(III) derived from experimental studies. Savenko (1998) estimated a stability constant for U(OH)3(aq) from a solubility constant provided in the literature for U(OH)3(s) (Lure, 1989). The reported solubility constant was log = 3.0, which led to a stability constant for U(OH)j(aq) of log = -32.6. There is no confirmatory evidence for these constants, and, as such, they are not retained. Moreover, given the smaller ionic radius of uranium(III) compared to that of actinium(III) (Shannon, 1976), the former ion should have a more positive log 3 and a more negative log which is not the case, providing support for the rejection of the data for uranium(III). 

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