Bond Length Definition:
“The distance between the nuclei of two atoms bonded together through a single covalent bond is called bond length.”
Bond Length Calculations:
C ─ C bond length in aliphatic hydrocarbons as well as in diamond is 154 pm (1.54 x 10-10 m)
In case of bond between identical atoms the bond length is additive property and can be obtained by adding atomic radii of the bonded atoms.
e.g. Atomic radius of carbon = 77 pm.
So C ─ C bond length = 77 + 77 = 154 pm.
Actual bond length of C ─ C bond = 154 pm.
In case of bond between un-identical atoms the bond length is not additive property and cannot be obtained by adding atomic radii of the bonded atoms. Due to difference of electronegativities a polar bond is formed between such atoms. And due to formation of positive and negative poles, atoms pull each other thus actual bond length is shortened.
e.g. (i) Bond length of Siδ+ ─ F δ– bond:
Atomic radius of Silicon = 117 pm.
Atomic radius of Fluorine = 64 pm.
So Si δ+ ─ F δ– bond length in SiF4 = 117 + 64 = 181 pm.
But actually Si δ+ ─ F δ– bond length in SiF4 = 155 pm.
e.g. (ii) Bond length of H δ+ ─ Cl δ– bond:
Atomic radius of Hydrogen = 37 pm.
Atomic radius of Chlorine = 99 pm.
So H δ+ ─ Cl δ– bond length in HCl= 37 + 99 = 136 pm.
But actually H δ+ ─ Cl δ– bond length in HCl = 127 pm.
Comparisons of experimental and calculated values of bond lengths and bond energies of Halogen acids: