By the end of this presentation, you should be able to: demonstrate a qualitative understanding of the  - particle scattering experiment and the evidence this provides for the existence, charge and small size of the nucleus.

 The Plum Pudding Model is an atom model proposed by JJ Thomson, the physicist who discovered the electron. You can easily picture it by imagining the plum pudding wherein the pudding itself is positively charged and the plums, dotting the dough, are the negatively charged electrons.  this model was invalidated in 1909 in what is now known as the Rutherford gold foil experiment, an experiment designed to probe the atom.  Rutherford expected very minimal alpha particle deflections as they were bombarded on the foil. Hence, the scintillations were only supposed to be observed right behind the foil, exactly opposite to the source of alpha particles. Alas, although majority of the flashes were indeed observed behind the foil, a few were also seen near the source.  According to Rutherford, “It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you.”

Scientists Geiger and Marsden carried out experiments by “shooting” positively charged particles at a thin sheet of gold foil. Thin sheet of gold foil Source of alpha particles, enclosed in a lead box. Counter to detect alpha particles. The counter can move around the grey table to different positions The entire experiment is carried out in an evacuated container so that there is nothing (other than the gold foil) to interfere with the alpha particles

They counted the largest number of particles when the detector was in line with the alpha source. Then they moved the detector. To their amazement, they actually found that some of the alpha particles came back very nearly in the same direction! A smaller number of particles were counted when the detector was in this position. Useful web site.

The alpha particles are continuously, but randomly emitted from the radioactive source. The particles passed through the gold foil and hit the screen which was coated with zinc sulphide. The screen glowed with a tiny bright spot wherever an alpha particle hit it. The actual experiment involves using a screen that glows, in order to detect the alpha particles after they pass through the foil.

Some of the particles were slightly deflected. This could be explained if we said that they could be repelled by the positive dough of protons. Since the protons were scattered around the atom, the force would be small.

The animation below shows the variety of paths an alpha particle may follow, depending on how close it comes to the nucleus. The force of repulsion between the positive nucleus and the positive alpha particle increases the closer they get.

The Plum Pudding Model could not explain the other observations!! Some were deflected by large angles and a few were sent back in the original direction! Yet in this model, there was no area where the charge was concentrated enough to produce large forces that would explain large deflections.

 The new model should be mainly space. This would explain how the majority of the positive alpha particles can go straight through, undeflected.  The new model should have a core with a large positive charge. This would then exert large forces of repulsion on the alpha particles depending on how close the particle came to the nucleus.  The electrons should be on the outside, around the nucleus.  The nucleus is tiny in comparison to the entire atom, this is why only a few alpha particles are sent straight back. These must have been projected straight toward the tiny positive core.

 Click here for an animation of a scattering experiment (remember that alpha particles do not hit the nucleus!)here for an animation of a scattering experiment