Ernest Rutherford described the result as the most incredible event of his life. It was, he said, almost as if you had fired a fifteen inch shell at a piece of tissue paper and it had come back and hit you.
The shell was an alpha particle. The tissue paper was a sheet of gold leaf a few thousand atoms thick. The fact that a small number of particles came straight back destroyed the accepted model of the atom and replaced it with one that survives, in its essentials, today.
The Model It Overturned
By 1900 the atom was known to have parts. J.J. Thomson had identified the electron in 1897 and shown that it is present in all matter, is negatively charged, and is roughly two thousand times lighter than a hydrogen atom.
Since atoms are electrically neutral, the negative electrons had to be balanced by positive charge. Thomson’s proposal, published in 1904, was that the electrons sit embedded in a sphere of positive charge spread evenly across the whole volume of the atom, like fruit in a pudding.
The model had a testable consequence. Positive charge diluted across the entire atom produces only weak electric fields. A fast, heavy, positively charged projectile fired at such an atom should pass through with a very slight deflection, a fraction of a degree at most. Firing many projectiles through many atoms would build up a small cumulative scatter, and nothing more dramatic.
The Experiment
Rutherford was working at Manchester, where he had established one of the strongest laboratories in Britain for the study of radioactivity. He had already shown that the radiation from radioactive elements comes in distinct types, naming the alpha and beta varieties in 1899, and had established that alpha particles are helium nuclei.
Alpha particles made an ideal probe: heavy, energetic, positively charged, and available in a steady supply from radium sources of the kind Marie Curie had spent four years isolating.
The work was carried out by Hans Geiger, Rutherford’s assistant, and Ernest Marsden, an undergraduate of twenty. The apparatus was simple:
- A radium source in a lead block with a narrow opening, producing a thin beam of alpha particles
- A sheet of gold foil, hammered to about four ten thousandths of a millimetre, roughly 8,600 atoms thick
- A movable screen coated in zinc sulphide, which emits a tiny flash of light when an alpha particle strikes it
- A microscope for observing the flashes
The counting was done by eye. An observer sat in a completely dark room for half an hour to adapt his vision, then watched through the microscope and counted scintillations, one at a time, for hours. Geiger later developed the electrical counter that carries his name largely to escape this procedure.
What They Found
Most alpha particles passed through the foil with little or no deflection, exactly as the pudding model predicted. Rutherford then suggested that Marsden check whether any were being deflected through large angles, a proposal he later admitted he made without expecting anything to come of it.
Marsden moved the screen around to the same side as the source and found flashes. Particles were coming back.
The proportion was tiny, roughly one in eight thousand deflected by more than ninety degrees, but the pudding model does not permit this at all. A diffuse cloud of positive charge cannot reverse a heavy fast particle any more than a fog bank can stop a bullet. Only a very concentrated charge, capable of exerting an enormous force at close range, can do it.
The 1911 Model
Rutherford spent nearly two years working out the implications before publishing. The delay was mathematical: he wanted to derive the exact distribution of scattering angles that a concentrated charge would produce, so that the prediction could be compared with the counts.
His conclusion, published in 1911, was that almost all the mass of the atom and all of its positive charge occupy a minute central region, with the electrons somewhere outside it. From the scattering data he could estimate the size of that region: about ten thousand times smaller than the atom itself.
Those proportions are hard to picture. If an atom were the size of a football stadium, the nucleus would be a pea at the centre circle. The rest is empty space occupied by electrons. Solid matter is, overwhelmingly, nothing at all.
The scattering formula Rutherford derived matched Geiger and Marsden’s counts across a wide range of angles, foil thicknesses, and metals. It was not a qualitative story but a quantitative fit, which is why it convinced people.
The Problem It Created
The nuclear atom immediately raised a difficulty that Rutherford could not solve.
If electrons orbit a central nucleus, they are accelerating, and classical electromagnetic theory is unambiguous about what an accelerating charge does: it radiates energy. An orbiting electron should lose energy continuously, spiral inward, and collide with the nucleus. Calculations put the lifetime of such an atom at around a hundred millionth of a second.
Matter exists, so something was wrong with the reasoning. The resolution came from a young Danish physicist who joined Rutherford’s group in 1912. Niels Bohr proposed in 1913 that electrons can occupy only certain permitted orbits, that they do not radiate while in them, and that light is emitted only when an electron jumps between orbits. The idea had no classical justification whatsoever, and it predicted the spectrum of hydrogen with startling accuracy.
Bohr’s rules were borrowed from Planck’s quantum hypothesis. The nuclear atom could not be made stable within classical physics, and its stability became one of the strongest arguments that classical physics needed replacing.
Rutherford’s Other Results
The gold foil experiment is the famous one, but it sits in the middle of a remarkable career.
Working with Frederick Soddy at McGill in Montreal, Rutherford established that radioactive decay is the transformation of one element into another, and that it follows an exponential law with a characteristic half life. This was an unwelcome conclusion at the time, since transmutation had been the goal of alchemy and chemists had spent a century establishing that elements are stable. The nucleus also explained why Mendeleev’s table works: what fixes an element’s place is the number of protons, not the atomic weight.
The work earned him the Nobel Prize in Chemistry in 1908, which amused and slightly annoyed him. He remarked that he had dealt with many transformations of various periods, but the quickest he had ever seen was his own from physicist to chemist.
In 1917 he produced the first artificial nuclear transmutation, bombarding nitrogen with alpha particles and converting it into oxygen. He also proposed the existence of the neutron, which his colleague James Chadwick found in 1932.
Like Thomson before him, he built a school. Twelve of his students and collaborators went on to win Nobel Prizes, including Chadwick, Blackett, Cockcroft, Walton, and Soddy.
Why the Experiment Is a Model
The gold foil work is taught in every physics course, and its structure explains why.
The prediction was specific. The pudding model did not merely feel vague, it forbade large angle scattering, and a single particle returning in the wrong direction was enough to falsify it. The observation was quantitative: Geiger and Marsden did not report that some particles bounced back, they counted how many at each angle. The replacement model was derived from the data rather than asserted, and it made further predictions that were then checked.
There is also a lesson about the value of looking where nothing is expected. Rutherford suggested the large angle search casually, as something to keep an undergraduate occupied. The measurement that mattered was the one nobody thought worth making.
The radioactive sources that made it possible came from the painstaking chemical work of the Curies, documented in Marie Curie’s 1903 doctoral thesis, which our bilingual edition presents with the French original and the English translation. Without a reliable supply of alpha particles there would have been no probe, and without a probe the interior of the atom would have stayed invisible.
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