Binding energy
The energy needed to pull something that is held together apart into its pieces. The larger it is, the more tightly the parts are bound.
A bound state has lower energy than its separated parts, and the difference is the binding energy. For a molecule it is what it takes to pull the atoms apart; for an atom, to pull an electron away from the nucleus; for a nucleus, to pull apart its protons and neutrons.
The binding energies of chemical bonds are a few electronvolts. Attempts to calculate the hydrogen molecule's value with quantum mechanics began in 1927, and in 1933 Hubert James and Albert Coolidge brought the gap with experiment down to 0.03 electronvolts[1]. In 2019 the energy needed to split the hydrogen molecule was measured to extremely high precision, in close agreement with theory[2].
Nuclear binding energies run to several million electronvolts per proton or neutron, roughly a million times those of chemical bonds. In the 1920s Francis Aston showed with precise mass measurements that nuclei are slightly lighter than the sum of their parts[3]. That difference corresponds to the binding energy, and it is the source of the energy in nuclear fusion and nuclear power.
Related terms
- Chemical bondThe link that holds atoms together in molecules and crystals, arising from electrons being shared or handed over.
- ElectronvoltThe unit of energy used for atoms: the energy one electron gains when accelerated through one volt. Its symbol is eV.
- Atomic nucleusThe tiny, heavy core at the centre of an atom, made of protons and neutrons and carrying a positive charge.
Sources
- H. M. James, A. S. Coolidge, “The Ground State of the Hydrogen Molecule,” J. Chem. Phys. 1, 825–835 (1933).
- N. Hölsch, M. Beyer, E. J. Salumbides, et al., “Benchmarking Theory with an Improved Measurement of the Ionization and Dissociation Energies of H2,” Phys. Rev. Lett. 122, 103002 (2019).
- F. W. Aston, “Bakerian Lecture. A new mass-spectrograph and the whole number rule,” Proc. R. Soc. Lond. A 115, 487–514 (1927).