Section 4.6: Thomson Model of the Atom

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Once the atomic nature of matter was on sound experimental and theoretical footing, the questioning then focused on the structure of atoms. Atoms were neutral, but contained negatively charged particles, electrons, that were too light (5.5 × 10−4 amu) to constitute the mass of the atom (the lightest atom, hydrogen, has a mass of 1 amu).  Thomson modeled the atom as made up of a uniformly charged sphere (positive) in which negative electrons were embedded and could move around. The charged sphere had a radius the size of an atom and was massive (in comparison with the electrons).  However, when alpha particles (positively charged particles with a mass of 4 amu) hit a thin foil of gold, some of them scattered through very large angles which was not predicted by Thomson's model.  Restart.

The animation shows what happens as a massive particle travels through a Thomson-like atom with light electrons. In this model, the particles interact by colliding with each other (hard-sphere collisions) which is not the same as the Coulomb interaction between the electrons and alpha particle. Nevertheless, the overall effect is the same. From conservation of momentum, the more massive alpha particle carries so much more momentum than the light electrons, the electrons barely influence its path. Certainly there is no way to have an alpha particle bounce backwards and in this animation, the yellow particle is only 200 times the mass of the electron instead of the actual 7000 times the mass. The electrons, then, do not noticeably alter the path of the yellow alpha particle.

Maybe, you might argue, instead of electron-alpha particle collisions, the Coulomb interaction between the alpha particle and the uniformly charged sphere could cause scattering. After all, the alpha particle would be repulsed by such a sphere.  The animation addresses this possibility.

Start.  To see the possible deflections, the animation has 20 alpha particles heading toward a positively charged sphere (indicated by the pink region). The alpha particles do not interact with each other, only with the charged sphere. Notice that again, the alpha particles' paths are not noticeably altered.5

The Thomson model, therefore, fails to account for an alpha particle hitting an atom and scattering through a large angle. Collisions between the incoming alpha particle and the electrons within the atom, in this model, are like a bowling ball rolling through plastic beads, while the alpha particle traveling through a positively charged sphere is like a bowling ball traveling through water instead of air. In neither case will the bowling ball recoil. But this is what Rutherford scattering showed: a bowling ball (alpha particle) occasionally recoils back towards the original direction of the throw as illustrated in Section 4.7.

5See K. Krane, Modern Physics, John Wiley, pp. 177-178 for an estimation of the average deflection of an alpha particle by a uniformly positively charged atom. For example, 5 MeV alpha particles scattered from a positively charged sphere with the positive charge of gold (Z = 79) and a radius the size of an atom, the average deflection is approximately 0.01o.