The negatively charged constituent of ordinary matter, responsible for the chemical properties of matter, and the carrier of electricity. The electron is the lightest known particle that possesses an electric charge. Its rest mass m_e is 9.1 × 10⁻³¹ kg (2.0 × 10⁻³⁰ lb), about 1/1836 of the mass of the proton or neutron, which are, respectively, the positively charged and neutral constituents of ordinary matter. The rest mass of an electron can also be expressed as 0.511 MeV/c², where c is the speed of light. Electrons were discovered in 1895 by J. J. Thomson in the form of cathode rays. The electron was the first elementary particle to be identified.