ARTICLE: Expanding the periodic table?

[John Jacobus <crispy_bird@YAHOO.COM>]

>From the Scientist, January 14, 2005



Expanding the periodic table?



Researchers create compounds from small clusters of

atoms acting like single atoms



By Philip Hunter

 

Whole new classes of compounds can be created from

small clusters of atoms acting like single atoms of

another element, researchers demonstrate for the first

time in work reported in this week's Science. Clusters

of 13 aluminum atoms behave like halogens and can

combine with them to form halogen compounds with novel

properties, the researchers write. And clusters of 14

aluminum atoms act like single alkaline earth atoms

capable of forming ionic compounds that are in effect

new types of salt.



This provides a compelling argument for redefining the

periodic table of elements, according to Shiv Khanna,

professor of physics at Virginia Commonwealth

University and co-leader of the study. "We call these

superatoms because I would classify [the 13-atom

aluminum clusters] in the same position as iodine,"

Khanna told The Scientist. "Since the iodine position

is already taken, I add a third dimension where [a

13-atom aluminum cluster] occupies the same row and

column as iodine but is above the plane of the

periodic table."



There is disagreement within the clustering field over

the significance of Khanna's work. "The idea that a

molecular fragment has the same properties as an atom,

ion, or molecule [of another element] is not new,"

said Andrew Barron, professor of materials science at

Rice University, who said that the idea first emerged

in 1976 in a paper on the isolobal principle,

describing how clusters could resemble atoms or ions

of other elements.



But the study brings together theory and experiment,

according to Kit Bowen, professor of chemistry at

Johns Hopkins University. "It's showing the chemical

generality of these clusters that's important," Bowen

told The Scientist. "The significance of the work is

that [aluminum cluster] ions can react with lots of

things to make molecules. It's one of the first steps

toward using clusters to assemble materials."



Khanna was one of the first to identify the potential

of such atomic clusters as building blocks for new

materials in 1995, according to Cowen, and the idea of

adding a third dimension to the periodic table started

to gather steam around that time.



Khanna's team also showed that a 14-atom aluminum

cluster could combine with iodine to yield iodide

salts. But just as there are differences as well as

similarities between elements within columns of the

periodic table, so the 14-atom cluster is not exactly

like any rare earth alkali, and therefore can be used

to create compounds with novel properties not found

before, Khanna said.



For example, when the 14-atom aluminum cluster

combines with three iodine atoms, it creates an ion

with overall negative charge which is actually

extremely stable. Such novel properties provide the

potential for creating radically new nanoscale

materials, Khanna said. He noted that the work on

clusters containing iodine could have important

medical applications, given the element's key role in

a number of biochemical pathways.



Links for this article

D.E. Bergeron et. al. "Al Cluster Superatoms as

Halogens in Polyhalides and as Alkaline Earths in

Iodide Salts," January 14, 2004:231-235, Science.

http://www.sciencemag.org 



Shiv N. Khanna

http://www.has.vcu.edu/phy/skhanna.htm 



M. Elian et al., "A comparative bonding study of

conical fragments," Inorg Chem, 15:1148, 1976

(Authorization required).

http://pubs.acs.org/cgi-bin/abstract.cgi/inocaj/1976/15/i05/f-p

df/f_ic50159a034.pdf 



Kit Bowen

http://www.jhu.edu/~chem/bowen/ 



S.N. Khanna, P. Jena, "Atomic clusters: Building

blocks for a class of solids," Phys Rev B Condens

Matter, 51:13705–16, May 15, 1995.

[PubMed Abstract]  

  

 

©2004 The Scientist, unless otherwise stated 





=====

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"It doesn't matter whether you're riding an elephant or a donkey if you're going in the wrong direction."

Jesse Jackson





-- John

John Jacobus, MS

Certified Health Physicist

e-mail:  crispy_bird@yahoo.com





		

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