(symbol e), fundamental physical constant expressing the naturally occurring unit of electric charge, equal to 1.6021892 × 10⁻¹⁹ coulomb, or 4.80325 × 10⁻¹⁰ electrostatic unit (esu, or statcoulomb). In addition to the electron, all freely existing charged subatomic particles (subatomic particle) thus far discovered have an electric charge equal to this value or some whole-number multiple of it. Quarks (quark), which are always bound within larger subatomic particles such as protons (proton) and neutrons (neutron), have charges of ¹/₃ or ²/₃ of this value.