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March 24, 2003
Volume 81, Number 12
CENEAR 81 12 p. 30
ISSN 0009-2347

ATOMIC DISTANCES
Linus Pauling's landmark paper in 1947 predicts metallic radii of most elements

ELIZABETH WILSON

8112pauling
MULTITALENTED Pauling, photographed in about 1960, teaches with molecular models. In the foreground is a model of the -helix structure he proposed for proteins, another of his great achievements.
Linus C. Pauling spent a good chunk of his career developing theories about chemical bonds. During the 1930s, he wrote a series of papers on the subject, culminating in his famous book "The Nature of the Chemical Bond." For the most part, his work back then focused on bonds that are covalent, such as those found in carbon tetrachloride.

But in 1947, he again broke ground, this time in his exploration of metallic bonds. The article "Atomic Radii and Interatomic Distances in Metals," published in the Journal of the American Chemical Society [69, 542 (1947)], predicted the metallic radii of most elements--illustrated in a periodic table. The article is the 79th most cited in the journal's 125-year history, underscoring the paper's lasting influence.

To be sure, Pauling's eminence imbued most of his work. "One of the reasons this paper is cited a lot is that everything Pauling did was cited a lot," says Kenneth W. Hedberg, emeritus chemistry professor at Oregon State University (OSU), Corvallis.

Hedberg was first a graduate student and then a researcher at California Institute of Technology while Pauling was chairman of the chemistry department there. Hedberg came to know Pauling and his family well. He explains how Pauling used his "marvelous chemical intuition" to describe the complexities of metallic bonding and to predict their radii.

An atomic radius can be envisioned as a hook sticking out from the center of the atom. When it grabs a hook on another atom, the combination of the two radii equals the bond length, Hedberg says.

Pauling theorized that bonds in metals are similar to some covalent bonds, in that they "resonate." In typical metallic lattices, atoms can take on a body-centered cubic structure with a nine-atom unit or a face-centered cubic structure with a 13-atom unit.

An atom such as, say, strontium, has two valence electrons available for bonding. But, explains Hedberg, an atom in the center of a body-centered cubic structure is surrounded by eight more atoms. The only way it can bind to all of them is to resonate, Pauling reasoned. An atom with more valence electrons can form stronger bonds, which are shorter, making the atomic radius smaller.

With equations derived from his work on covalent bonds, Pauling predicted the bond lengths for the metallic radius of an atom in the center of a face-centered cubic unit (coordination number 12). He also predicted the metallic radii for the case of a single atom bound to another.

PERIODIC A segment of Pauling's table of metallic atomic radii shows the atoms' valence (v), the metallic radii of the atoms with a coordination number of 12 in a face-centered cubic lattice [R(CN12)], and the metallic radii of the atoms singly bonded to one other atom [R(1)].

Pauling was born in 1901, in Portland, Oregon. He attended Oregon Agricultural College in Corvallis, which would later become OSU, and obtained his bachelor's degree in chemical engineering in 1922. He then went to Caltech for graduate school, and in 1928 joined the faculty there as an assistant professor of theoretical chemistry. Pauling's career was extraordinarily diverse, and included work on X-ray crystallography and the structure of DNA. In 1954, he won the Nobel Prize in Chemistry "for his research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances."

Pauling's wife, Ava Helen, was a peace activist who urged him to take up the cause. His outspoken antiwar stance landed him in trouble with the government and Caltech trustees. But Pauling won another Nobel Prize, this time for peace, in 1962. In 1963, he left Caltech to join the faculty of Stanford University.

Later in his life, Pauling became a proponent of the healing powers of vitamin C and founded the Pauling Institute for Medical Research in Palo Alto, Calif. In 1986, Pauling bequeathed his and his wife's personal and professional memorabilia to OSU (C&EN, Aug. 7, 2000, page 62). Pauling donated about 10,000 to 15,000 items to OSU each year until his death in 1994.

Pauling's friends and colleagues also remember him as a gifted communicator, as his JACS paper on metallic radii of the elements demonstrates.

"I took great pleasure in rereading this article," Hedberg says. "He wrote just as he spoke--it flowed right out of him." 

 


C&EN is celebrating the 125th volume of the Journal of the American Chemical Society by featuring selected papers from among its 125 most cited. This paper was ranked 79th.




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