Electron mass

J. J. Thomson measured e/m for cathode rays in 1897 and found a particle nearly two thousand times lighter than hydrogen, which is how the electron was discovered — as a ratio, not a mass. Only when Millikan pinned down e could the mass itself be extracted. Today the electron mass is known to 3 parts in 10¹⁰ from Penning-trap cyclotron-frequency comparisons against a bare carbon nucleus.

In the units physicists actually use, m_e = 5.485 799 090 44 × 10⁻⁴ u = 0.510 998 950 69 MeV/c² — that 511 keV is the energy of each annihilation photon a PET scanner detects, and the threshold for pair production is twice it. Because the electron is 1836 times lighter than the proton, it moves fast enough to define chemistry while the nucleus sits nearly still (the Born-Oppenheimer approximation), and the reduced-mass correction to hydrogen's spectrum is a tidy 1 part in 1836.