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Physical Chemistry

New And Improved Covalent Atomic Radii

April 28, 2008 | A version of this story appeared in Volume 86, Issue 17

An updated set of atomic radii has been generated for the elements hydrogen through curium, an accomplishment that fills in gaps and resolves inconsistencies in a pool of data currently used to study chemical structure, bonding, reactivity, and periodic trends (Dalton Trans., DOI: 10.1039/b801115j). The covalent radius, one of chemistry's earliest structural benchmarks, is a measure of the size of an atom when it participates in a covalent bond. The sum of the radii of the two atoms that make up a bond serve as a good estimate of the bond length. Santiago Alvarez and coworkers at the University of Barcelona sought to develop a comprehensive set of standardized radii and carried out their study by analyzing 228,000 experimental bond distances mined from the Cambridge Structural Database. The researchers first refined the radii for carbon, nitrogen, and oxygen, which are the most commonly used elements. They then determined values for all other elements based on bond distances to C, N and O. Although the data show few surprises, the study provides "a useful, incremental refinement of statistically based covalent radii," note Clark R. Landis and Frank A. Weinhold, two bonding experts at the University of Wisconsin, Madison.


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