Actinium physical properties

Electronic Structures. Almost all the physical properties and chemical behavior of the rare earth elements find a logical explanation in terms of their electronic structures. Scandium, yttrium, lanthanum, and actinium are the first members, respectively, of the first, second, third, and fourth transition sequences of elements. In other words, each such element marks the beginning of an inner building where a stable group of 8 electrons is expanding to a completed (or more nearly complete) group of IS. This situation is illustrated for the first transition sequence. 

The actinoid series encompasses the fourteen chemical elements with atomic numbers from 90 to 103, thorium (Th) to lawrencium (Lr). The actinoid series derives its name from the group-IIla element actinium (Ac) which can be included in the series for the purpose of comparison. Only Th and uranium (U) occur in usable quantities in nature. The other actinoids are man-made elements. Pure Th is a silvery-white metal which is air-stable and retains its luster for several months. U exhibits three crystallographic modifications as follows a (688°C) —> P (776°C) —> U is a heavy, silvery-white metal. The luster of freshly prepared americium (Am) is white and more silvery than neptunium (Np) or plutonium (Pu) prepared in the same manner. All actinoid elements are radioactive. Table 2.113 sutnmarizes some physical properties of actinoid metals (Th, U and Am). 

These two kinds of lead are now known to be isotopes, or inseparable elements which belong in the same space in the periodic table and yet differ in atomic weight and in radioactive properties. According to Frederick Soddy, the first clear recognition of isotopes as chemically inseparable substances was that of H. N. McCoy and W. H. Ross in 1907 (75,107). Strictly speaking, the science of radioactivity has revealed only five naturally occurring new elements with distinctive physical and chemical properties polonium, thoron, radium, actinium, and uranium X2. All the other natural radioactive elements share previously occupied places in the periodic table. 

The elements in the group III B scandium, yttrium, lanthanum and actinium that have an incompletely filled d subshell in their atomic state (n - l)d ns. Although both lanthanum and actinium could be included in the d transition metal series, they are very similar physically and chemically to the elements in the f-block and therefore are considered to be f-type transition elements (4f-, 5f-type transition elements, respectively). The last element of the lanthanides series, lutetium, also has a partly filled d orbital (Table 2.6) and could also be included in the d transition metal group. However, it has similar properties to the 4f-type transition metals, where it is usually grouped with lanthanum and the rest of the lanthanides series. 

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