A new set of covalent atomic radii that is more comprehensive and precise than previous lists has been compiled by Spanish scientists from the University of Barcelona.

The concept of atomic radii is used in structural chemistry and X-ray crystallography to give a rough idea of the size of an atom in a molecule or crystal, and to establish bonding and non-bonding interactions between atoms. The new list of radii covers elements up to curium (atomic number 96) and shows sensible trends along both the rows and groups of the periodic table, explains team leader Santiago Alvarez.

The work began when Alvarez's team were trying to decide when to expect structures to have a through-ring metal-metal interaction, and discovered that existing lists of radii contained gaps and inconsistencies. They found periodic trends were erratic and that in the previously most complete set used by the Cambridge Structural Database (CSD) important metals such as nickel, palladium, platinum and gold were given default unrefined values of 1.50 Å.

The Spanish team derived their radii from a systematic search of 228 000 experimental bond distances in structures held in the CSD and the Inorganic Crystal Structure Database. They determined the radii from bond distances to nitrogen, carbon and oxygen atoms, as there many examples of covalent bonds from these to most other elements.

Michael Townsend