Bond energy is a term used in chemistry to describe the amount of energy required to separate the chemical bonds between atoms. It is defined as the amount of heat energy required to break a specific quantity of bonds of one type and is expressed as kilojoules per mole of bonds (kj/mol). A mole is a constant, equal to 6.02 x 1023 atoms or molecules of a particular substance. The bond energy of a particular bond depends on the type of bond, and some are much stronger than others. Ionic bonds, formed by the transfer of electrons from one atom to another are generally the strongest, and hydrogen bonds are the weakest.
Some chemists and texts refer to the energy needed to break bonds apart as bond dissociation energy, sometimes expressed as a negative value, and the energy needed to form the bonds as bond energy, expressed as a positive value, but this is mainly a matter of semantics as the absolute amounts are identical for any given bond. The same amount of energy is released when bonds are formed as must be applied to break them. This can cause confusion, but the terms bond dissociation energy and bond energy are sometimes used interchangeably. The main difference is the usage of one or the other to describe what kind of reaction is happening, not the amount of energy involved.
The type of bond is not the only determining factor for bond energy. For example, a single ionic bond between one pair of atoms may have a different bond energy than a single ionic bond between another pair of atoms of different types. Ionic bonds tend to be stronger than other bonds and have a higher bond energy, but the strongest covalent bonds, in which atoms share pairs of electrons, may have higher bond energy than the weakest of ionic bonds.
Chemists use bond energy to calculate specifics about certain chemical reactions. They can also use archived data about the bond energies of specific bonds to predict the way certain chemical reactions will behave and the amount of energy that will be released or that may need to be added to achieve a certain reaction. Bond energies for multiple bonds are sometimes combined when talking about the cumulative bond energies of complex compounds with multiple chemical bonds.