(b) Nagaoka proposed that atoms resembled the planet Saturn, with a ring of electrons surrounding a positive “planet.” (credit a: modification of work by “Man vyi”/Wikimedia Commons credit b: modification of work by “NASA”/Wikimedia Commons) The term “electron” was coined in 1891 by Irish physicist George Stoney, from “ electric i on.”įigure 2.8 (a) Thomson suggested that atoms resembled plum pudding, an English dessert consisting of moist cake with embedded raisins (“plums”).

Although 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 more than one thousand-times less that of an atom. (credit a: modification of work by Nobel Foundation credit b: modification of work by Eugen Nesper credit c: modification of work by “Kurzon”/Wikimedia Commons)īased on his observations, here is what Thomson proposed and why: The particles are attracted by positive (+) charges and repelled by negative (−) charges, so they must be negatively charged (like charges repel and unlike charges attract) they are less massive than atoms and indistinguishable, regardless of the source material, so they must be fundamental, subatomic constituents of all atoms. Simultaneous deflections by applied electric and magnetic fields permitted Thomson to calculate the mass-to-charge ratio of the particles composing the cathode ray. (c) In the cathode ray, the beam (shown in yellow) comes from the cathode and is accelerated past the anode toward a fluorescent scale at the end of the tube. (b) This is an early cathode ray tube, invented in 1897 by Ferdinand Braun. Thomson produced a visible beam in a cathode ray tube.

The results of these measurements indicated that these particles were much lighter than atoms ( Figure 2.6).įigure 2.6 (a) J. In similar experiments, the ray was simultaneously deflected by an applied magnetic field, and measurements of the extent of deflection and the magnetic field strength allowed Thomson to calculate the charge-to-mass ratio of the cathode ray particles. This beam was deflected toward the positive charge and away from the negative charge, and was produced in the same way with identical properties when different metals were used for the electrodes. When high voltage was applied across the electrodes, a visible beam called a cathode ray appeared between them. This apparatus consisted of a sealed glass tube from which almost all the air had been removed the tube contained two metal electrodes.

If matter is composed of atoms, what are atoms composed of? Are they the smallest particles, or is there something smaller? In the late 1800s, a number of scientists interested in questions like these investigated the electrical discharges that could be produced in low-pressure gases, with the most significant discovery made by English physicist J. Define isotopes and give examples for several elements.Describe the three subatomic particles that compose atoms.Summarize and interpret the results of the experiments of Thomson, Millikan, and Rutherford.Outline milestones in the development of modern atomic theory.By the end of this section, you will be able to:

Development of the atomic theory