Glossary / History of physics
Electronvolt
The unit of energy used for atoms: the energy one electron gains when accelerated through one volt. Its symbol is eV.
In joules, the everyday unit of energy, the energies involving a single atom are awkwardly tiny, so this unit is used instead. One electronvolt is 1.602176634×10⁻¹⁹ joules. Since the electron's charge was fixed by definition in 2019, this value is exact too[1].
For a sense of scale: one particle of visible light carries about 2–3 electronvolts, pulling the electron off a hydrogen atom takes about 13.6 electronvolts, and chemical bonds are a few electronvolts. Nuclear reactions are measured in millions of electronvolts (MeV); one D-T fusion reaction releases 17.6 MeV.
Related terms
- Binding energyThe energy needed to pull something that is held together apart into its pieces. The larger it is, the more tightly the parts are bound.
- Planck constantThe fundamental constant, written h, that sets the step size of energy for light and matter. Frequency times h is the smallest unit of energy for that light.
- Photoelectric effectThe ejection of electrons from a metal when light shines on it. The energy of each electron depends on the light's frequency (colour), not its intensity.