Rutherford's Experiment - Part II: The Paper of 1911

Go to Part I- the pre-history of Rutherford Experiment from 1906 to early 1911

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I. What Confronted Rutherford?

Ernest Rutherford had been studying alpha particles since 1898. In fact, he discovered them. To him, alpha particles were part of the family. In 1909 he was confronted with some rather bizzare alpha-particle behavior that he had to explain. What was the behavior, exactly?

Hans Geiger and Ernest Marsden aimed a stream of alpha particles at a thin gold foil for several months in 1909. (They would continue studying scattering until 1913.) Geiger cites a thickness of 8.6 x 10¯6 cm. for the foil. In fact, the foil was so thin that it had to be supported on a glass plate. (The plate without any foil was studied and no deflecions were found. It was transparent to the alpha particles.) There were three major findings:

1) Almost all of the alpha particles went through the gold foil as if it were not even there. Those alpha particles, of course, continued on a straight-line path until they hit the detector screen.

2) Some of the alpha particles were deflected only slightly, usually 2° or less. Geiger found that an alpha particle was, on average, deflected about 1/200th of a degree by each single encounter with a gold atom. The most probable angle of deflection for one gold foil turned out to be about 1°. (Rutherford cites a figure of 0.87° in his 1911 paper.)

3) A very, very few (1 in 8000 for platinum foil) alpha particles were turned through an angle of 90° or more. (Rutherford cites 1 in 20,000 for gold in his 1911 paper.)

This is a diagram incorporating the three findings. R is the source of alpha particles and F is the foil that scatters the alpha particles. M is the microscope used to look at the detector screen which was attached to the front of the microscope.

The flashes on the screen were very faint, so a very dark room was required. The person doing the viewing had to sit in the dark for about an hour before beginning the experiment, to ensure maximum eye sensitivity.

All Rutherford had to do was explain how it all fit together.

II. OK, Get to the Answer, Big Guy

And Rutherford was a big guy. He was fun, outgoing and vigorous, the life of the party -- a great, big, over-grown child. He was also a hard-working scientist who loved science for itself and never tired of playing in the laboratory. He seldom had problems with people; on one occasion was even able to turn a potential enemy into a co-worker. Upon meeting people like Einstein, Lorentz, and Planck for the first time, he was able to turn them into immediate best buddies. He found a real soul-mate in Marie Curie. They both loved doing pure research, just letting the science take them where it would, with no purpose other than to discover new and exciting things.

His solution to the enigma of explaining both large- and small-angle scattering, as you probably know, was the nucleus. It was already well established that the atom had a radius of about 10¯8 cm. The Thomson model of the atom spread the entire mass of the atom throughout that space. What Rutherford did was put most of the mass of the atom at the center of the atom, in a space much, much smaller that the atom itself -- this is the nucleus.

For the purposes of his 1911 paper, he considered the nucleus to act as a point:

"We shall suppose that for distances less that 10¯12 cm the central charge and also the charge on the alpha particle may be supposed to be concentrated at a point."

Rutherford never used the word "nucleus" in his paper. His phrase was "charge concentration." In 1912, in a book he published, he devotes a few pages to the nuclear model and uses the word nucleus once.

So, how does the nucleus account for the three major findings by Geiger and Marsden?

1) The nucleus is so small that the odds are overwhelmingly in favor of a given alpha particle motoring right on through the gold foil as if nothing were there. It turns out that the atom is a very empty place, indeed!

2) Some alphas, by pure random chance, will pass near some gold atom nuclei during their passage through the foil and will be slightly deflected. By pure chance, some or all of the small deflections will add up and shove the alpha particle off a straight-line path. Those alphas will emerge slightly deviated (say one or two degrees) from a straight-line path. (It might be helpful to remember that the gold nucleus and the alpha particle are both positively charged, so they will repel each other as they come close together.)

3) A very, very few alphas, by pure, random chance, will hit a nucleus almost head-on. The alpha, traveling at 10% the speed of light, penetrates the atom and gets very close to the nucleus. However, the repulsion between the alpha and the atom nucleus is so great that the atom flings the alpha back out, and it does so in a hyperbolic path. Depending on various factors, this occasionally results in the alpha being turned around 90° or more. The very heavy nucleus recoils a bit from the impact, but essentially goes nowhere.

III. That's It, Folks

Rutherford closed the door on the basic structure of the atom. No serious challenge has arisen to the nuclear model of the atom. However, early in his paper, Rutherford writes the following:

"The question of the stability of the atom proposed need not be considered at this stage, for this will obviously depend upon the minute structure of the atom, and on the motion of the constituent charged parts."

Rutherford is not prepared to take the next step, which is to determine how the electrons are arranged in the atom. However . . .

In March 1912, 27-year-old Niels Bohr (awarded a Ph.D. in May 1911, the same month of Rutherford's classic paper) will arrive in Rutherford's laboratory, having just spent a bit more than 6 months in J.J. Thomson's laboratory. But that is another story for another day. Ernest Rutherford has discovered the nucleus and now it's time to take a well-earned rest.

Go to Part I- the pre-history of Rutherford Experiment from 1906 to early 1911

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