The distance between atomic centers involved in a chemical bond. 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-diffraction 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 spectroscopy, 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 bond number (the Pauling's bond order).