The distance between atomic centers involved in a @CT07009@. The notion of bond length is defined differently in various experimental methods of determination of molecular geometry; this leads to small (usually \(0.01{-}0.02\ \mathring{\text{A}}\)) differences in bond lengths obtained by different techniques. For example, in gas-phase electron-@D01711@ experiments, the bond length is the interatomic distance averaged over all occupied vibrational states at a given temperature. In an X-ray crystal structural method, the bond length is associated with the distance between the centroids of electron densities around the nuclei. In gas-phase microwave @S05848@, the bond length is an effective interatomic distance derived from measurements on a number of isotopic molecules, etc. A number of empirical relationships between bond lengths and bond orders in polyatomic molecules were suggested, see, for example, fractional @B00705@ (the Pauling's @B00707@).