Bond Energy

Bond energy is the strength of a chemical bond between atoms, expressed as the amount of energy required to break it apart. It is as if the bonded atoms were glued together: the stronger the glue is, the more energy would be needed to break them apart. A higher bond energy, therefore, means a stronger bond.

Bond energies are usually expressed in kilojoules per mole (kJ/mol): the number of kilojoules of energy that it would take to break apart exactly one mole of those bonds is 6.02 × 10 ²³. There are several kinds of "glues," or attractions, by which atoms and molecules can stick together. Table 1 shows the approximate ranges of their strengths, from the strongest to the weakest.

Notice that ionic bonds are stronger than covalent bonds. Among covalent bonds, triple bonds are stronger than double bonds and double are stronger than single bonds. Hydrogen bonds are weaker than all, but they play a big role in determining the properties of important compounds such as proteins and water.

Bonds of the same type can vary quite a bit in their strengths. The bond energies of several specific bonds are shown in this table.

Bond energies between certain pairs of atoms vary somewhat, depending on the particular molecule they are part of, because adjacent atoms can affect their bonding slightly. The values in Table II are average bond energies for the listed bonds.