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Furthermore, they are less massive than atoms and indistinguishable, regardless of the source material, so they must be fundamental, subatomic constituents of all atoms.Īlthough controversial at the time, Thomson’s idea was gradually accepted, and his cathode ray particle is what we now call an electron, a negatively charged, subatomic particle with a mass approximately two thousands that of a hydrogen atom.
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The term “electron” was coined in 1891 by Irish physicist George Stoney, from “ electric ion.”Ĭlick here to hear Thomson describe his discovery in his own voice. In 1909, more information about the electron was uncovered by American physicist Robert A. Millikan created microscopic oil droplets, which could be electrically charged by friction, set to drop due to gravity, and then slowed or even reversed by an applied electric field lower in the apparatus. By adjusting the electric field strength and making careful measurements and appropriate calculations, Millikan was able to determine the charge on individual drops ( Figure 2), and figure out the charge on individual electrons. Further work with charge to mass ratios allowed him to determine the mass of the electron as well. Scientists had now established that the atom was not indivisible as Dalton had believed.īut the structure of the atom was otherwise not well understood. Where did these electrons reside? And what about the positively-charged portions of the atoms that also must exist to counterbalance the negative electron charge?