In chemistry, bond energy (BE), also called the mean bond enthalpy or average bond enthalpy is the measure of bond strength in a chemical bond. IUPAC defines bond energy as the average value of the gas-phase bond-dissociation energy (usually at a temperature of 298.15 K) for all bonds of the same type within the same chemical species. The enthalpy of formation ΔHfº of a large number of atoms, free radicals, ions,clusters and compounds is available from the websites of NIST,NASA, CODATA, and IUPAC. Most authors prefer to use theBDE values at 298.15 K.