Question

Define the bond length.

Answer 1

Bond length is defined as the equilibrium distance between the nuclei of two bonded atoms in a molecule.
Bond lengths are expressed in terms of Angstrom \((10^{ -10}\; m)\) or picometer \((10 ^{-12}\; m)\) and are measured by spectroscopic X-ray diffractions and electron-diffraction techniques. 
In an ionic compound, the bond length is the sum of the ionic radii of the constituting atoms \((d = r_+ + r_-)\). In a covalent compound, it is the sum of their covalent radii \((d = r_A + r_B)\).