0.3 Structure of an atom  (Page 3/5)

The experimental data for the scattering of the α particles shows three primary results, as depicted in [link] . First, and perhaps most surprisingly, the greatest number of the α particles pass directly through the gold foil without any deflection in their paths. Second, a much smaller number of the α particles experience small deflections in their paths. And third, a very small fraction of the α particles (perhaps 1 in 50,000 or more) are deflected back in the direction of the beam they came from.

For now, we’ll focus on the first and third observations. It is surprising that the largest number of the α particles pass through the gold foil as if it is not there. In fact, it is just as if the gold foil consisted mostly of empty space, and most of the α particles seem to pass through that empty space. This seems strange, since the gold foil is certainly solid and doesn’t appear to be empty space. The third result gives a striking contrast to this result. A tiny fraction of the α particles must encounter something other than empty space. To rebound, an α particle must hit something much more massive than itself. We might think that this is a gold atom, which is much heavier than an α particle, but to be consistent with the first observation, most of the gold atom must be empty space.

Thinking about these two observations leads us to a simple model for the atom. Each gold atom must be mostly empty space and most of the mass of the gold atom must be concentrated into a very small fraction of the volume of the atom. We will call this concentrated mass the “nucleus.” A careful calculation based on the fraction of α particles which pass through and the fraction which rebound tells us that the diameter of the nucleus is about 100,000 times smaller than the diameter of the atom itself. This is an amazing result! Although an atom is very small, the nucleus is very much smaller than that, as analogously depicted in the diagram in [link] .

This is an interesting model that seems to account for two of our three observations, but it is a puzzling model too. If the mass of the atom is concentrated in such a small space, what occupies the rest of the volume of the atom? The second observation from the experiment helps us understand this. Since a small number of the positively charged α particles are deflected in their paths, this suggests that they come close to but do not run into something positive in the atom. Since these α particles are deflected, whatever they come close to must be more massive than they are. This means that they are coming close to the nucleus and the nucleus must be positively charged.

This gives us a clue about what occupies the vast empty space of the atom. If the positive charges of the atom are concentrated in the nucleus, the negative charges in the atom, the electrons, must be in the much larger space of the atom outside of the nucleus. This nuclear model of the atom then accounts for all three observations in the α particles gold foil scattering experiment. Most of the volume of the atom is empty space in which negatively charged electrons move about. Most of the mass and all of the positive charge of the atom is concentrated in a nucleus, which is tiny in comparison to the atom.