Bond energy

In chemistry, bond energy (E) is the measure of bond strength in a chemical bond. It is the heat required to break one mole of molecules into their individual atoms. For example, the carbon-hydrogen bond energy in methane E(C–H) is the enthalpy change involved with breaking up one molecule of methane into a carbon atom and 4 hydrogen radicals divided by 4.
Bond energy (E) should not be confused with bond-dissociation energy. It is a roughly transferable property, and enthalpy of formation can typically be roughly approximated by simply adding tabulated values for bond energies for all bonds in a molecule, with an error of sometimes just a few percent. However, to get a better approximation is much more difficult.

Bond energy/distance correlation

Bond strength (energy) can be directly related to the bond length/bond distance. Therefore, we can use the metallic radius, ionic radius, or covalent radius of each atom in the molecule to determine the bond strength. For example, the covalent radius of boron is estimated at 83.0 pm, but the bond length of B–B in B₂Cl₄ is 175 pm, a significantly larger value. This would indicate that the bond between the two boron atoms is a rather weak single bond. In another example, the metallic radius of rhenium is 137.5 pm, with a Re–Re bond length of 224 pm in the compound Re₂Cl₈. From this data, we can conclude that the bond is a very strong bond or a quadruple bond. This method of determination is most useful for covalently bonded compounds.