Figure 1. Roll over different parts to see the different results that Geiger and Marsden might have seen.
|The evolution of the atomic model|
| ||The early models of the atom were based on the evidence that Dalton and Thomson had at the time. Thomson suggested the plum pudding model.|
|Rutherfords new evidence allowed him to propose a more detailed model with a central nucleus. However, this was not the end of the story. |
|Neils Bohr's famous model with orbiting electrons. However, Bohrs atom would give out electromagnetic waves and lose energy, causing the electron to spiral into the nucleus.|
|In the 1920s, a whole new theory of physics, called quantum mechanics, presented an even more radical picture of the atom. The electrons cannot be pinpointed but exist as a sort of cloud of probability outside the nucleus. |
This model of the atom allowed physicists to develop lasers and semiconductors and produce the information and communication technology that we rely on today.
| ||Geiger and Marsden fired the alpha particles at a target made from gold foil. They chose gold because it could be beaten into a very thin sheet only a few atoms thick. The whole apparatus was sealed inside a vacuum because alpha particles are deflected by air particles. The alpha particles were detected as small flashes on a fluorescent screen. Geiger and Marsden counted the flashes as they looked down a telescope tube at the screen. They moved the telescope around the target to see how many alpha particles were deflected in each direction.|
What they saw on the fluorescent screen.
|An incredible event|
| ||The results of the experiment were totally unexpected. The most extraordinary discovery was that some of the alpha particles were scattered back the way they came. Rutherford commented:|
| ||"It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you had fired a fifteen inch shell at a piece of tissue paper and it came back and hit you." |
The other main result was that more than 99.99% of the alpha particles were hardly deflected at all.
|Analysing the results|
| ||Rutherfords analysis changed the way that we think of an atom. Up until that time scientists thought of the atom as a tiny but solid object. Rutherford concluded that: |
|New theory |
|How he knew |
|most of the atom is most of the empty space || |
because most of the alpha particles were undeflected
|the positive charge of the atom is concentrated in a tiny nucleus || |
he used mathematics to work out how much charge would be needed on the nucleus to give different results. He changed the values in his calculations until they matched the real results
|the nucleus has a large mass || |
the nucleus had to be massive to make the alpha particles bounce back; if it were not massive then the alpha particles would simply knock it out of the way
He worked out that the diameter of the nucleus is about 100,000 times smaller than the diameter of the atom.
|The story so far|
| ||Rutherford found that the atom contains a tiny nucleus|
| ||The nucleus has all the positive charge of the atom|
| ||The nucleus has most of the mass of the atom|
| ||The diameter of the atom is 100,000 times more than that of the nucleus|| || |