In HF the experimental bond length value is less than the sum of covalent radii. Explain why.
The energy of each bond formed by two different atoms (H-F) has two parts: E = Ec + Ei , where Ec – the energy conditioned by the covalency of the bond and Ei – the energy conditioned by the electrostatic attraction between atoms. The covalent radii are obtained by calculating the distance between equal atoms (H-H or F-F) when Ei = 0. Therefore, if Ei is not counted, the experimental distance differs from the calculated one. It becomes shorter because the bound atoms with different electronegativities have the opposite charges (Hδ+ and Fδ-). Thus, they are attracted to each other increasing the energy of the bond, which means the bond becomes shorter.