Electronvolt (in joules)

A joule is absurdly large for a single particle, so atomic, nuclear and particle physics run on the electronvolt: numerically identical to e, because W = qV. Visible photons are 1.6–3.3 eV, chemical bonds a few eV, X-rays tens of keV, nuclear binding energies around 8 MeV per nucleon, and the LHC collides protons at 6.8 TeV each. Silicon's 1.12 eV band gap is why sunlight redder than about 1100 nm produces no photocurrent at all.

Two conventions catch people out. First, hc = 1239.84 eV·nm is the shortcut worth memorising — a 620 nm photon is exactly 2 eV. Second, "MeV" for a mass really means MeV/c²; the electron's 0.511 MeV is its rest energy, and you must divide by c² before putting it in a Newtonian formula. Since e became exact in 2019 the eV is an exact multiple of the joule, though it remains outside the SI proper.