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In chemistry, bond energy (E) or (Mean, Average) bond enthalpy (H) is the measure of bond strength in a chemical bond. IUPAC defines bond energy as the average value of the gas-phase bond dissociation energies (usually at a temperature of 298 K) for all bonds of the same type within the same chemical species. For example, the carbon–hydrogen bond energy in methane H(C–H) is the enthalpy change involved with breaking up one molecule of methane into a carbon atom and four hydrogen radicals, divided by 4. Tabulated bond energies are generally values of bond energies averaged over a number of selected typical chemical species containing that type of bond. Bond energy (E) or (Mean, Average) bond enthalpy (H) should not be confused with bond-dissociation energy. Bond energy is the average of all the bond-dissociation energies in a molecule, and will show a different value for a given bond than the bond-dissociation energy would. This is because the energy required to break a single bond in a specific molecule differs for each bond in that molecule. For example, The H2O molecule is composed of two O-H bonds bonded as H-O-H. The bond energy for H2O is the average of energies required to break the two O-H bonds of the H2O molecule. Although the two bonds are the same, the energy values slightly differ because the first O-H bond is broken from H-OH structure whereas the second bond is broken as .O-H. Since an oxygen atom is more electronegative than a hydrogen atom, breaking O-H bond from water molecule is influenced by the presence or absence of hydrogen atoms on both sides of oxygen atom. Therefore, the average is taken as the bond energy.

Bond energy table Calculation Equation

ΔH = ∑ ΔH(bonds broken) - ∑ ΔH(bonds formed)

This basically means that you add up all the energies of the broken bonds; add up all the energies of the bonds that are reformed and subtract one from the other. Example Find H for the following reaction given the following bond energies:

Bond	 	Bond Energy (kJ/mol) H-H		436 O=O		499 O-H		463 We have to figure out which bonds are broken and which bonds are formed. 2 H-H bonds are broken. 1 O=O bond is broken 2 O-H bonds are formed per water molecule, and there are 2 water molecules formed, therefore 4 O-H bonds are formed Now we can substitute the values given into the equation:

Bond energy–distance correlation
Bond strength (energy) can be directly related to the bond length and bond distance. Therefore, we can use the metallic radius, ionic radius, or covalent radius of each atom in a 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.

Factors affecting ionic bond energy
There are several contributing factors but usually the most important is the difference in the electronegativity of the two atoms bonding together.