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Atomic Structure
The Discovery of ...
... the atomic center.
Discovery of the atomic center

"Scattering is the devil", Rutherford once wrote in a letter to a friend. He was talking about a phenomenon he and his co-worker, Hans Geiger, had seen in one of their experiments.

When they fired alpha particles through a very, very thin sheet of gold foil Geiger began to notice "notable" scattering of the rays, which prevented him from getting clear pictures at the other end of the tube. Their apparatus was very similar to the one shown below.

A source of alpha rays (and particles) is attached to one end of a glass tube from which all the air is pumped. The alpha rays pass easily along the tube to the other end where they crash into molecules of zinc sulfide. Every time a crash occurs a tiny flash of light is given off, and this flash (called a "scintillation"), can be seen and recorded by someone looking through a small microscope. Naturally the room has to be very dark, and the experimenters had to sit in the dark room for hours so their eyes could adjust.

alpha rays pass though gold

Even when a thin sheet of gold foil is placed in the path of the rays, most of them pass through it very easily. One or two of the rays, however, are scattered left, right, up and down making the pattern of scintillations seen at the other end of the tube much more "fuzzy". This was the "notable scattering" that Has Geiger was seeing.

Something was causing the rays to scatter, and Rutherford and Geiger wanted to know what it was. They tried other metals and found that aluminum did not scatter as much or as far, but then realized that most of the scattering was coming from the walls of the tube! It was an artifact that could be cleared up by modifying the tube to include baffles, or washers that prevented the rays bouncing off the walls.

But not completely! Despite all their precautions, some rays were definitely being nudged out of their straight-line path by something in the metal foils. What was going on?

a new assistant

All laboratories get new assistants from time to time, and Rutherford and Geiger's laboratory was no exception. Twenty year old Ernest Marsden joined the team in about 1908/1909, and needed something to do. As happens a lot in such situations, the head of the laboratory (Geiger and Rutherford) scratch their heads and ask themselves what an untrained assistant can do to occupy his time while they get on with the more serious stuff.

In this case Rutherford threw out a whacky idea. "See if he can get some effect from alpha particles directly reflected from the metal surface... Huh?" he remembered saying to Geiger and Marsden, and then walking out of the laboratory. Behind him he left the other two wondering if it were possible for alpha particles to strike the surface of the gold foil and then be bounced back (reflected) in the roughly same direction. How could they find out?

Marsden did not think that Rutherford was really expecting any results, but went along with the hunch anyway and built a small piece of equipment that would provide an answer. It is shown in the figure below.

A source of alpha particles is hidden behind a lead screen so that the rays cannot impact directly on the scintillation screen and be observed. Then a piece of gold foil is placed in the chamber in such a way that any reflected alpha rays bounce back and can be seen flashing on the scintillation screen.

alpha rays are reflected!

With no gold foil in place, nothing was seen on the screen, but when the foil was inserted Geiger reported to Rutherford in great excitement that "We have been able to get some of the alpha particles coming backwards!". It was an unexpected and very important discovery, which Rutherford started to think about. What did it mean?

About 1 in 20,000 alpha particles shot at a thin foil of gold were reflected back just as light does when falling on a mirror. Marsden and Geiger tested other metals, and got the following results:

many other metals do the same
Metal Atomic mass flashes/minute
Lead 207 62
Gold 197 67
Platinum 195 63
Tin 119 34
Silver 108 27
Copper 64 14.5
Iron 56 10.2
Aluminum 27 3.4

The larger the atomic weight, the more alpha particles that were reflected.

By December 14th, 1910, in a letter to B.B. Boltwood, Rutherford had sorted though a number of possible explanations for these "reflections" of alpha particles, and wrote that he was "... doing a good deal of calculation on scattering". His thoughts were also beginning to come together.

explain these facts

Three facts had to be explained:

  • The vast majority of alpha particles pass straight though a piece of metal foil as if it was not there.
  • Some alpha particles are deflected (scattered) by an angle of about 1o as they pass through the metal foil.
  • About 1 alpha particle in 20,000 (for gold) hits something and bounces back (is being reflected).

It was reasonably well known that the radius of a single atom (in the gold) was about 10-8 cm. If the mass of the atom were evenly spread out across this distance there would not be much empty space either inside the atom or between atoms. But... and here came Rutherford's "big" idea ...

the nuclear atom

... if most of the mass of an atom was concentrated in a very dense lump in the middle of the atom (he called it the "charge concentration"), and the rest of the atom were empty space, then ...

All was explained!

  • The dense atomic centers (each later called a "nucleus") of the gold atoms were so small that most of the alpha particles zoom right through as if there was nothing there.

  • Occasionally a positively charged alpha particle will come close to one of these dense atomic centers, which are also positively charged. This close encounter between two similar electric charges pushes the rays off their straight line path. This causes a small deflection in the direction taken by the alpha particle as it leaves the gold.

  • A tiny number of alpha particles, traveling at 10% of the speed of light, hit a dense atomic center right in its middle. The collision and the repulsion cause the alpha particle to "bounce" backwards and move on a very different path. These are the reflected rays.

This then was the first true picture of an atom; a small very dense, positively charged center constituting the vast majority of the mass, and about 1/100,000 th the diameter of that of the whole atom. Around this center the lighter, negatively charged electrons filled the rest of the volume of the atom, but contributed almost nothing to the mass.

Now, only two things needed further clarification; where and how were the electrons arranged, and why did the number of positive units in the dense center of the atom (what we now know are the protons) not equal the known total mass (weight of) the "nucleus"?

© 2003, Professor John Blamire