binding energy The energy released when protons and neutrons bind together to form an atomic nucleus, or the energy required to break up that nucleus. The binding energy per proton or neutron is greatest for atoms with mass numbers in the range 50–65 (the iron peak). Energy is released when lighter atoms combine to form heavier ones (as in nucleosynthesis inside stars), provided the products are no heavier than iron. Conversely, when heavy atoms break down to lighter ones (e.g. in fission reactors) energy is also released, provided the products remain heavier than iron. The binding energy is the equivalent of the mass defect.

Bond energy

KEY TERMS

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. There are several kinds of “glues,” or attractions, by which atoms and molecules can stick together. Depending upon the type of attractive force, the bond energy can vary in strength. For example, an ionic bond, which is a simple interaction between a positively-charged group and a negatively-charged group, is stronger than a type of bond called a covalent bond, which involves the sharing of electron between the atoms participating in the bond. Among covalent bonds, triple bonds, which involve the sharing of three electrons, are stronger than double bonds and double bonds are stronger than single bonds. Hydrogen

KEY TERMS

Kilojoule— An amount of energy equal to a thousand joules. One kilojoule is equivalent to 0.239 kilocalorie. In electrical terms, a kilojoule is the amount of energy used by one kilowatt of power operating for one second.

bonds are weakest of all these bonds. However, in a molecule there can be many individual hydrogen bonds, so their total strength can be considerable. Hydrogen bonds play an important role in determining the properties of important compounds such as proteins and water.

See also Dipole.

Robert L. Wolke