Intorduction. The Actinide and Transactinide Elements.

The actinide group consist of following elements: Actin­ium - [Ac], Thor­ium - [Th], Protac­tinium - [Pa], Ura­nium - [U], Neptu­nium - [Np], Pluto­nium - [Pu], Ameri­cium - [Am], Curium - [Cm], Berkel­ium - [Bk], Califor­nium - [Cf], Einstei­nium - [Es], Fer­mium - [Fm], Mende­levium - [Md], Nobel­ium - [No], Lawren­cium - [Lr].

The “actinides” (“actinons” or “actinoids”) are the fourteen elements from thorium to lawrencium inclusive, which follow actinium in the periodic table. They are analogous to the lanthanides and result from the filling of the 5f orbitals, as the lanthanides result from the filling of the 4f. The position of actinium, like that of lanthanum, is somewhat equivocal and, although not itself an actinide, it is often included with them for comparative purposes.

Prior to 1940 only the naturally occurring actinides (thorium, protactinium and uranium) were known; the remainder have been produced artificially since then. The "transactinides" are still being synthesized and so far the nine elements with atomic numbers 104-112 have been reliably established. Indeed, the 20 manmade transuranium elements together with technetium and promethium now constitute onefifth of all the known chemical elements. In 1789 M. H. Klaproth examined pitchblende, thought at the time to be a mixed oxide ore of zinc, iron and tungsten, and showed that it contained a new element which he named uranium after the recently discovered planet, Uranus. Then in 1828 J. J. Berzelius obtained an oxide, from a Norwegian ore now known as “thorite”; he named this thoria after the Scandinavian god of war and, by reduction of its tetrachloride with potassium, isolated the metal thorium. The same method was subsequently used in 1841 by B. Peligot to effect the first preparation of metallic uranium. The much rarer element, protactinium, was not found until 1913 when K. Fajans and O. Gohring identified 234Pa as an unstable member of the 238U decay series:

92238U ( -α ) → 90234Th ( -β ) → 91234Pa ( -β ) → 92234U

They named it brevium because of its short half-life (6.70 h). The more stable isotope 231Pa (T1/2 = 32760 y) was identified 3 years later by O. Hahn and L. Meitner and independently by F. Soddy and J. A. Cranston as a product of 235U decay:

92235U ( -α ) → 90231Th ( -β ) → 91231Pa ( -α ) → 89227Ac

As the parent of actinium in this series it was named protoactinium, shortened in 1949 to protactinium. Because of its low natural abundance its chemistry was obscure until 1960 when A. G. Maddock and coworkers at the UK Atomic Energy Authority worked up about 130 g from 60 tons of sludge which had accumulated during the extraction of uranium from UO2 ores. It is from this sample, distributed to numerous laboratories throughout the world, that the bulk of our knowledge of the element’s chemistry was gleaned.

In the early years of this century the periodic table ended with element 92 but, with J. Chadwick’s discovery of the neutron in 1932 and the realization that neutron-capture by a heavy atom is frequently followed by, femission yielding the next higher element, the synthesis of new elements became an exciting possibility. E. Fermi and others were quick to attempt the synthesis of element 93 by neutron bombardment of 238U, but it gradually became evident that the main result of the process was not the production of element 93 but nuclear fission, which produces lighter elements. However, in 1940, E. M. McMillan and P. H. Abelson in Berkeley, California, were able to identify, along with the fission products, a short-lived isotope of element 93 (T1/2 = 2.355 days):

92238U + 01n → 92239U ( -β ) → 93239Np

As it was the next element after uranium in the now extended periodic table it was named neptunium after Neptune, which is the next planet beyond Uranus. The remaining actinide elements were prepared by various "bombardment" techniques fairly regularly over the next 25 years though, for reasons of national security, publication of the results was sometimes delayed. The dominant figure in this field has been G. T. Seaborg, of the University of California, Berkeley, in early recognition of which, he and E. M. McMillan were awarded the 1951 Nobel Prize for Chemistry.

The isolation and haracterization of these elements, particularly the heavier ones, has posed enormous problems. Individual elements are not produced cleanly in isolation, but must be separated from other actinides as well as from lanthanides produced simultaneously by fission. In addition, all the actinides are radioactive, their stability decreasing with increasing atomic number, and this has two serious consequences. Firstly, it is necessary to employ elaborate radiation shielding and so, in many cases, operations must be carried out by remote control. Secondly, the heavier elements are produced only in the minutest amounts. Thus mendelevium was first prepared in almost unbelievably small yields of the order of 1 to 3 atoms per experiment!