Question #54369

In HF the experimental bond length value is less than the sum of covalent radii. Explain why.

Expert's answer

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.

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.

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