By the end of this presentation you should be able to…… RADIOACTIVITY: MODELS OF THE ATOM By the end of this presentation you should be able to…… Describe simply the Plum Pudding model (put forward by Thomson) and the nuclear model (put forward by Rutherford) of the atom. Explain how the observations made in the Rutherford scattering experiment exposed flaws in the Plum Pudding model and led to the development of the nuclear model (which is still used today).

The Plum Pudding Model The Nuclear Model MODELS OF THE ATOM The Plum Pudding Model The Nuclear Model

THE FIRST MODEL THE PLUM PUDDING MODEL It was first thought that positive charge and electrons were all mixed up. Very much like a plum pudding. The negative electrons were like the plums, randomly scattered in a positive “dough”. Positive charge surrounds the electrons, like the dough of the pudding Negative electrons (like the plums) Note the absence of neutrons, they were not yet discovered!

Experimentation reveals flaws in the Plum Pudding Model! Scientists Geiger and Marsden carried out experiments by “shooting” positively charged particles at a thin sheet of gold foil. 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 Thin sheet of gold foil Counter to detect alpha particles. The counter can move around the grey table to different positions Source of alpha particles, enclosed in a lead box.

The Rutherford Scattering Experiment Useful web site. A smaller number of particles were counted when the detector was in this position. 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!

The actual experiment involves using a screen that glows, in order to detect the alpha particles after they pass through the foil. 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.

Some of the particles were slightly deflected Some of the particles were slightly deflected. This could be explained if we said that they could be repelled by the positive dough. Since the positive charge was spread throughout the atom, the force would be small.

The Plum Pudding Model could not explain all the other observations!! THE PROBLEM The Plum Pudding Model could not explain all 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 positive charge was concentrated enough to produce large forces that would explain large deflections. Most went straight through, if the atom was filled with protons and electrons, shouldn’t most (if not all) of the positive alpha particles be deflected by a small amount? No holes were left! (!!!???)

THE SOLUTION Throw out the old model and introduce a new one that will explain all the observations of the experiment! 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 positive core. 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.

If the alpha particle (+) passes very far from the nucleus (also +), it will go straight through. The closer it gets to the nucleus, the greater the force of repulsion from the nucleus, and the more it will be pushed away. On the rare occasion that the alpha particle comes straight towards the tiny nucleus, a large repulsive force will push it straight back. +

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.