Relativity Could Alter Superheavy Bonding

For decades, chemists have hypothesized that relativistic effects could alter the bonding properties of superheavy elements. These effects come into play when the charge on an atom’s nucleus becomes so large that its electrons orbit close to the speed of light. A new theoretical study from a team led by Jun Li at Tsinghua University adds more intrigue to the idea, suggesting that diatomic molecules made of superheavy group VI elements have a lower bond order than their lighter group VI counterparts (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.5b11793). For example, although group VI diatomic molecules such as Cr₂, Mo₂, and W₂ sport a hextuple bond, the Tsinghua group’s model predicts that the superheavy molecule Sg₂ will have only a quadruple bond. The study also suggests a similar bond order reduction for group VII superheavy diatomic molecules. The conclusions can’t be tested with today’s technology—superheavy elements have lifetimes on the order of seconds at most and are therefore challenging to characterize. The work “provides a rare example where relativistic effects break the periodicity in the nonrelativistic domain,” the authors say.