Seaborgs Actinide Concept

It was Seaborg who proposed the actinide concept it has become generally accepted practise to call the 15 elements beginning with Ac the actinide elements. However, we have to notice that, although immediately following lanthanum the 4f orbitals become more favourable than the 5d for the entrance of further electrons in the following elements there is not (until later) a similar situation for the 5f-6d... 

When G. T. Seaborg in 1944 introduced his actinide concept the theory played not the last role in his decision to place newly discovered elements in a second series where the filling of the 5f-shell takes place, similarly to the lanthanide series where the filling of the 4f-shell takes place. Thus, the filled-shell concept was in accord with the newly found periodicity in chemical properties and resulted in the discovery of all the heavy actinides at that time [20]. 

SDS. See Sodium dodecyl sulfate Seaborg s actinide concept, 75 Secondary... 

Seaborg, G. T. 1994. Origin of the actinide concept. In Handbook on the physics and chemistry of rare earths. Vol. 18, Ixmlhanides/actinides chemistry, ed K. A. Gschneider, L. Eyring, G. R. Choppin, and G. H. Lander, pp. 1-27. New York Elsevier Science. 

O Table 18.10 shows ionic radii of actinide elements together with those of lanthanide elements (Seaborg and Loveland 1990). The usefiil data on the ionic radii and coordination number are given by Shannon and Prewitt (1969). They carried out comprehensive study of crystal, or ionic, radii by analyzing the crystal structures of many fluoride, oxide, chloride, and sulfide compounds. Marcus published a data book on the properties of ions (Marcus 1997). The book covers a wide range of information on ionic radii of the actinide elements and other ions. Ionic radii of actinide elements decrease with increasing atomic number. This behavior is called actinide contraction and is one of the important examples of the actinide concept. 

About three years later (in 1949), I published the following account of the status of the actinide concept (Seaborg 1949). 

The following seven elements, completing the actinide series as predicted at element 103, were synthesized and identified (i.e., discovered) during the following twelve years (i.e., by the year 1961). Their chemical properties conformed very well with those predicted on the basis of the actinide concept. Elements 104 and 105, the first elements beyond the actinide series, were later also shown to have the predicted chemical properties. I summarized the situation in my book, Man-Made Transuranium Elements (Seaborg 1963), as follows. 

Element 94 was named plutonium after the planet discovered last, Pluto. In 1941, the first 0.5 /rg of the fissionable isotope Pu were produced by irradiating 1.2 kg of uranyl nitrate with cyclotron-generated neutrons. In 1948, trace amounts of Pu were found in nature, formed by neutron capture in uranium. In chemical studies, plutonium was shown to have properties similar to uranium and not to osmium as suggested earlier. The actinide concept advanced by G. T. Seaborg, to consider the actinide elements as a second / transition series analogous to the lanthanides, systematized the chemistry of the transuranium elements and facilitated the search for heavier actinide elements. The actinide elements americium (95) through fermium (100) were produced first either via neutron or helium-ion bombardments of actinide targets in the years between 1944 and 1955. 

Shortly after the discovery of transuranic elements about 50 years ago (see Morss and Fuger 1992), Seaborg suggested that the heaviest elements form a 5f transition series, which came to be known as the actinides, in analogy to the well known 4f series of the lanthanides. A glance at a modern periodic system of elements shows this basic concept to be still accepted, despite the fact that pronounced differences... 

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