Actinium, 433 - halides

In principle, a promising method for the preparation of Ac metal is the tantalothermic reduction of AcC, as described generally in Section II,C. This method has not been tried as yet, however, so the metallothermic reduction of an actinium halide or oxide remains the only proved method. 

Lawrencium is the last of the transuranic elements and the 15 th in the actinide series (there are 15 elements in the lanthanide series as well, assuming you start counting the series at the elements lanthanide and actinium, respectively.) It is assumed that lawrencium has some chemical and physical characteristics similar to lutetium, located just above it in the lanthanide series. It is also located at the bottom of the group 17 (VILA) elements, which makes it the heaviest of the halides. 

The general chemistry of Ac3 in both solid compounds and solution, where known, is very similar to that of lanthanum, as would be expected from the similarity in position in the Periodic Table and in radii (Ac3, 1.10 La3, 1.06 A) together with the noble gas structure of the ion. Thus actinium is a true member of Group 3, the only difference from lanthanum being in the expected increased basicity. The increased basic character is shown by the stronger absorption of the hydrated ion on cation-exchange resins, the poorer extraction of the ion from concentrated nitric acid solutions by tributyl phosphate, and the hydrolysis of the trihalides with water vapor at 1000°C to the oxohalides AcOX the lanthanum halides are hydrolyzed to oxide by water vapor at 1000°C. 

Actinium forms halides only in the (+3) oxidation state, AcXs and AcOX (X = F, Cl, Br). The trihalides have the LaFs (X = F) and UCI3 (X = Cl, Br) structures, respectively. 

In aU known compounds, actinium appears as Ac with the radon structure, the hydrated ion occurring in aqueous solutions of its salts. Thorium salts give Th + ions. The identification of halides of Th and Th Ms questioned solids resulting from the thermal decomposition of Thl4 liberate hydrogen from water, the thorium being oxidised to Th, presumably, if a lower iodide is formed, by the reaction ... 

Isomorphism among compounds of the actinides is common and only a few examples need be given. The dioxides, MO2, of thorium, uranium, neptunium, plutonium and americium all have a fluorite lattice. The trihalides of the transuranic elements are isomorphous not only with the corresponding trihalides of actinium and uranium but also with those of the lanthanides. Isomorphism is also exhibited in many complex halides thus thorium, ura-... 

Little need be said about the halides of actinium. All of the binary halides of this element have been prepared by conventional procedures, and the straightforward, uncomplicated chemical behavior that actinium exhibits generally is also evident here. Thorium, however, presents quite a different problem. Thorium gives no indication of existence in aqueous solution in an oxidation state lower than +4. From considerations of the... 

The metallothermic reduction of halides was the first method to be successfully applied. Actinium metal can... 

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). 

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