https://www.iitutor.com In 1909, at Ernest Rutherford’s suggestion, German physicist Hans Geiger and Ernest Marsden, his English undergraduate student, used alpha (α) particles from a radium source to bombard thin metal foils in an evacuated cylindrical chamber. The metal foil and a radium alpha particle source were kept in a fixed position. To ensure the alpha particles were travelling towards the screen in a straight path, a slit system was developed that would block stray alpha particles. Geiger and Marsden obtained results that indicated a very high degree of alpha particle penetrability through the apparently solid foil. The alpha particles were not directly seen to penetrate the foil because they were far too small. Evidence for this happening was obtained through observation of a screen within the chamber. The alpha particle source was pointed at the slits that stopped the stray alpha particles not aimed at the centre of the screen F. The thin metal foils M were inserted, with the result that the deflection of the a-particles could be detected by a moveable microscope and screen. The experimental apparatus used a microscope with a scintillating screen over the objective lens that could be turned through any desired angle. When an alpha particle hit the screen there was a flash. The set-up of the equipment enabled the scientists to move the microscope to any angle, and to count the number of flashes produced on the screen as deflected alpha particles struck it. Using the apparatus, Geiger and Marsden were able to determine the relative probability of alpha particles being deflected through angles as great as 90° at the foil. The pattern of deflections they observed without curve A and with the metal foil inserted curve B suggested that there was space within the solid material. Without the metal foil inserted, most of the alpha particles hit the centre of the screen or close to it. Some deflections of the alpha particles were always expected because of the inability to remove all gas from within the evacuated tube. Because of this, occasional collisions and deflections of the alpha particles were to be expected. When the metal foil was inserted, the deflection pattern altered to one similar to shown in curve B with some particles being deflected from hitting the centre of the screen. This occurred presumably after the alpha particles hit atoms in the foil. Many alpha particles still passed straight through the foil as shown by the curve. Since microscopic holes were not evident in the solid foil, it became obvious that another explanation must be found for those alpha particles that passed straight through the solid metal foil. Rutherford took the deflection data results with a similar distribution curve to B and performed statistical analysis on it. He then asked Geiger and Marsden to move the microscope around to the alpha particle source side of the apparatus to see what the probabilities of various degrees of backscatter were. Geiger and Marsden found the backscatter. This was assumed to be due to alpha particles being reflected back at the source by some solid object much more massive than the alpha particles. Rutherford’s Analysis Rutherford's theoretical statistical analysis of the scattering distribution measured by Geiger and Marsden showed that it was consistent with an atom with a solid nucleus of radius of only about 10–14 m. This small value for the diameter of the nucleus meant that the solid centre of the atom was only a tiny fraction of the atom's total volume, but it confirmed that the nucleus contained most of the atom's mass. Rutherford's Atom In 1911, Rutherford suggested an atomic model in which electrons orbited a tiny positive nucleus. The nucleus contained the vast majority of the atom’s mass. He proposed that the electrostatic attraction between the positive nucleus and the negative electrons provided- the centripetal force needed to keep the electrons in their orbits. Calculations and later scattering experiments indicated that the radius of the nucleus was less than 1 / 10 000 th of the radius of the atom. Experiments by Rutherford and others showed that the atomic number (Z), which was used position elements on the periodic table, was actual the number of fundamental positive charges in the nucleus. Thus hydrogen had one positive charge in the nucleus and one orbiting electron, helium had two positive charges in the nucleus and two orbiting electrons, lithium three positive charges, and so on. While Rutherford’s model was a step forward, it was criticised by other physicists because the orbiting charges radiate electromagnetic waves, and therefore the electrons should lose energy and spiral into the nucleus. In addition, the model could not be used to explain the emission specific frequencies of light (the atomic spectra) from excited atoms.

• published: 18 Mar 2016
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