electronvolt (eV)
/ ih-LEK-tron-volt; eV = ee-VEE /
The joule, the everyday science unit of energy, is comically huge for a single particle — like measuring the weight of an eyelash in tonnes. Particle physicists needed a unit scaled to the things they study, so they chose one tied to a simple, concrete event: how much energy a single electron picks up when pushed across the voltage of a one-volt battery. That tiny amount is one electronvolt, written eV.
By definition, one electronvolt is the energy gained by one electron moving through a potential difference of one volt. It is a very small energy in human terms (about 1.6 times ten-to-the-minus-nineteen joules), but it is just right for the subatomic world: the energy holding an electron in a hydrogen atom is about 13.6 eV, and the light your eyes see carries a couple of eV per photon. Because mass and energy are interchangeable, the same unit doubles as a unit of mass (an electron's mass is about 500,000 eV) and, with the speed of light folded in, of momentum too — a convenience that comes from using natural units.
The electronvolt matters because it is the native currency of the field: every particle's mass, every collision energy, every binding energy gets quoted in eV or its multiples. Speaking the same unit lets a physicist instantly compare, say, the energy of a chemical bond (a few eV) with the energy needed to make a Higgs boson (125 billion eV). One caution to keep straight: an eV is a unit of energy, not of voltage. A one-volt battery and a one-electronvolt energy are related but are different kinds of quantity.
A single photon of red light carries about 1.8 eV of energy; a photon of blue light, being more energetic, carries about 3 eV. That small difference in electronvolts is the entire difference between the colours your eye perceives.
The electronvolt is sized for the subatomic world — even visible light is just a few eV.
An electronvolt measures energy, not voltage. By mass-energy equivalence the same unit is also used for mass (often written eV/c-squared), so saying a particle weighs so many eV is shorthand, not a confusion of units.