Radioactive transition metal of Group III. Atomic number 89. Symbol Ac. All isotopes arc radioactive; atomic weight tables list the atomic weight as [227], the mass number of the most stable isotope.
Actinium is exclusively tripositivc and resembles the tripositivc rare earth elements in its chcmical properties. It forms insoluble compounds of the same type as the latuhanide elements, such as the fluoride and oxalate. The hydroxide is also insoluble. The similarities to the lanthanidcs appear in the crystallization of double salts, such as, for example, with magnesium nitrate, where actinium follows the lanthanides and is very difficult to separate from them. Differences from the lanthanides arc to be found in the extent to which complcx ions are formed, actinium being, in general, less easily subject to complcx ion formation than any of the lanthanidc elements. This is presumably related to the basic or clcctropositivc charactcr of actinium, a consequence of its larger ionic radius, and it appears that actinium is more basic than even lanthanum. Actinium goes with cerium group of rare earths in those separations in which the yttrium group is separated with the help of complcxing agents. The successful separation of the lanthanidc elements from cach other by use of the ion exchange resins is also applicable to the separation of actinium from the lanthanidc elements and the heavier tripositivc actinidc elements.
Except for the sulphide, the compounds of actinium arc colourless. All of the pure compounds of actinium which have been prepared and whose structures have been determined are isostuctural with the analogous lanthanidc and actinidc compounds and in cach the actinium is tripositivc. In addition to the solid halides and oxyhalidcs, a number of other compounds such as scsquioxidc, sulphide, phosphate, oxalate, and a double salt with potassium sulphate have been prepared.
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