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 $(d = r_A + r_B)$.