Mendeleev symposium focuses on research on extending table, understanding periodicity C H E M ED, HISTORY, AMD INORGANIC
The periodic arrangement of elements that Dmitri I. Mendeleev disclosed to the scientific world 100 years ago has been added to and refined in many ways. Articles describing the types of periodicity that occur in the periodic table and attempts to explain why these periodicities occur abound in the chemical literature. Yet the search for a consistent set of periodic rules that would apply to all exploratory studies in inorganic chemistry continues. This was made evident in the current research described at the Symposium on a Century of Chemical Periodicity: The Mendeleev Centennial, at the 157th ACS National Meeting in Minneapolis, Minn. The symposium was held jointly by the Divisions of Chemical Education, the History of Chemistry, and Inorganic Chemistry. One of the spectacular prospects for the periodic table is in extending it far beyond the number of elements known today (C&EN, April 2 1 , page 13 ). Dr. Glenn T. Seaborg, Chairman of the U.S. Atomic Energy Commission, proposes an extension all the way to element 168. Using computer techniques developed at Los Alamos Scientific Laboratory, he proposes that elements 104 to 112 are formed by the filling of the 6d subshell. Elements
113 through 118 result from the filling of the 7p subshell. The addition of electrons to the 8s subshell forms elements 119 and 120, and the next electron added is the first of the 7d subshell, resulting in element 121. Dr. Seaborg, as well as other chemists, speculates that an inner transition series of elements, similar to the lanthanide and actinide series, should begin somewhere around element 120. Quantum theory indicates that this series would be formed by the addition of 18 electrons to an inner 5g subshell or 14 electrons to a 6f subshell, but the order of filling these shells could not be predicted until the Los Alamos computer techniques became available, he says. The computer calculations suggest that, after the addition of some electrons to the 7d and 6f subshells, the filling of the 5g subshell takes place in an orderly manner. The AEC chairman notes that it is reasonable to expect that this would be followed by the filling of the inner 6f subshell, leading to an inner transition series of 32 elements ending with element 153. The lower members of the series might be generally homologous with the lower members of the actinide series, perhaps through about element 124, Dr. Seaborg says. Throughout the series, however, the correlation would be indistinct although the tripositive oxidation state might be the distinctive one. In further describing some proper-
ties of the proposed series, Dr. Seaborg points out that since the difference in energy levels of successive electrons would be very small, the series would exhibit multiple, barely distinguishable oxidation states, leading to very complicated chemistry. H e refers to the 32-element series as the superactinide series because of the rough analogy to the actinide and lanthanide series. However, he notes, each element of the proposed series does not correspond to an actinide or lanthanide element on a one-for-one basis, considering that there are only 14 elements in either the lanthanide or actinide series. Beyond the superactinides, he speculates, addition of electrons to the remaining positions in the 7d subshell would form elements 154 through 162. Filling the 8p subshell of six electrons would result in elements 163 through 168. In attempting to formulate predictive laws for relative nuclear stability, Dr. Seaborg notes, nuclear physicists indicate that there may exist certain combinations of neutrons and protons—closed nucléon shells—in superheavy nuclei that may be stable enough to permit experimental observation. In the absence of such closed shells, he says, all superheavy elements would decay by spontaneous fission with half-lives so short that scientists could neither produce nor observe them. Nuclides with closed shells should be resistant to decay by spontaneous fission, and hence their rates of decay by alpha- and betaparticle emission become of great importance. Theoretical considerations by sci-
lllliij»^
SHBjri^y^^ 1l l î É v^HI lllieilllii^^pl'' ' IP?!^^|pPs:X"
flKjlfar • AEC chairman Glenn T. Seaborg (left) proposes extending the periodic table to element 168, far beyond its current form or the version presented by Dmitri Mendeleev one hundred years ago
| j
'1
XII1
m^^ÊÊf1 ^Ê^^^Êf^:: y·-^' ^β'Ι^ΒΗ^^βΛίΐ ' fi i:iï ^ ^ ^ β Ι ' Η •||||: B Κ- Iff! -il? ?\&ZMÎWÊI^BÈÈÈÊË ;
1!!I-:BH
®
SWi*
«88888888883888? ;ί*ίί:
i l l WË ^^,,g^^ 'l!lll§|:,lï | f
mm
V>^Ly^^Ê M
^il^^^^ii^ llliliiliP : :ίί
Ik/- aw Itli'ifllll
Λ
^ •
