Rutherford Model

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Rutherford's assistants did all the work. Rutherford's idea was, "well let's see if these guys (Geiger, Marsden etc.are good at detecting alpha particles scattered from gold foil. Make them look for them at large angles. That sounds like a really difficult task." But then, Geiger and Marsden actually found particles scattered at extreme angles.

Experiment -- alpha particles bombarding gold foil (polonium a source)

A small fraction of the α-particles falling upon a metal plate have their directions changed to such an extent that they emerge again at the side of incidence.

Compared, however, with the thickness of gold which an α-particle can penetrate, the effect is confined to a relatively thin layer. In our experiment, about half of the reflected particles were reflected from a layer equivalent to about 2 min. of air. If the high velocity and mass of the a-particle be taken into account, it seems surprising that some of the α-particles, as the experiment shows, can be tamed within a layer of 6 × 10⁻⁵ cm. of gold through an angle of 90°, and even more. To produce a similar effect by a magnetic field, the enormous field of 10⁹ absolute units would be required.

Three different determinations showed that of the incident α-particles about 1 in 8000 was reflected, under the described conditions.

On a Diffuse Reflection of the α-Particles. Hans Geiger, John Harling Fellow, Ernst Marsden. Proceedings of the Royal Society 82 (1909): 495-500.

It was left to Rutherford to make conclusions from their observations

most particles did not deviate	atoms are mostly empty space
some deviated slightly	there's something positive inside the atom
a tiny fraction deviated nearly 180°	a small positively charged region (nucleus) contains most of the atom's mass
	electrons orbit the nucleus like a planet orbiting the sun

Rutherford's own words.

The observations, however, of Geiger and Marsden on the scattering of a rays indicate that some of the α particles, about 1 in 20,000 were turned through an average angle of 90 degrees in passing though a layer of gold-foil about 0.00004 cm. thick, which was equivalent in stopping-power of the a particle to 1.6 millimetres of air.

It seems reasonable to suppose that the deflexion through a large angle is due to a single atomic encounter, for the chance of a second encounter of a kind to produce a large deflexion must in most cases be exceedingly small. A simple calculation shows that the atom must be a seat of an intense electric field in order to produce such a large deflexion at a single encounter.

Consider an atom which contains a charge ±Ne at its centre surrounded by a sphere of electrification containing a charge ±Ne [N.B. in the original publication, the second plus/minus sign is inverted to be a minus/plus sign] supposed uniformly distributed throughout a sphere of radius R. e is the fundamental unit of charge, which in this paper is taken as 4.65 x 10⁻¹⁰ E.S. unit. We shall suppose that for distances less than 10⁻¹² cm. the central charge and also the charge on the alpha particle may be supposed to be concentrated at a point. It will be shown that the main deductions from the theory are independent of whether the central charge is supposed to be positive or negative. For convenience, the sign will be assumed to be positive. 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.

In comparing the theory outlined in this paper with the experimental results, it has been supposed that the atom consists of a central charge supposed concentrated at a point, and that the large single deflexions of the α and β particles are mainly due to their passage through the strong central field.

Ernest Rutherford. "The Scattering of α and β Particles by Matter and the Structure of the Atom." Philosophical Magazine (6) 21 (May 1911): 669-688.

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Resources

general
- Ernest Rutherford -- Scientist Supreme, John Campbell
- Physics 2000, University of Colorado
- The Mechanical Universe and Beyond (video on demand, login required)
  - The Atom, A history of the atom, from the ancient Greeks to the early 20th century, and a new challenge for the world of physics

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