In 1910, Ernest Rutherford oversaw Geiger and Marsden carrying out his famous experiment. Rutherford knew he could not ever see inside an atom using visible light, because its wavelength is too long. However, he realised that he could probe inside the atom using a particle that is smaller than the atom. He chose alpha particles because:
although at the time, it was unclear what they were, he knew they were very small
they were released by radioactive atoms and he suspected that they might be deflected (he had noticed their deflections in air)
unlike beta particles, all the alpha particles from a given source have the same energy.
Picture 1.4 The Geiger and Marsden apparatus. Roll over the picture at different angles to show what they might have seen in picture 1.4a.
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 by the foil in each direction.
"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."
They found that while most of the alpha particles passed through the foil, a small number were deflected. And, to their surprise, some alpha particles bounced straight back.
More than 99.99% of the alpha particles were hardly deflected at all.
Picture 1.4a What they saw
on the fluorescent screen. Roll over picture 1.4 to see what they might have seen at each angle.
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:
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.
Picture 1.5 The electrons are whizzing about outside the nucleus. They are not necessarily going in orbits.
Our model of the atom
There have been a number of theories before and since Rutherford to describe the structure of the atom (see below). Our current model of the atom is still based very much on Rutherford's ideas.
There are electrons on the outside of the atom. They have very little mass (less than a thousandth of the mass of the smallest atom). Most of the mass of the atom is in a central nucleus whose diameter is about 1015m. This is about 100,000 times smaller than the diameter of the atom. You can think of it as being like a pinhead in the middle of an athletics stadium: the pinhead is the nucleus; spectators are the electrons.
We now think that the electrons form a cloud around the nucleus. Their exact position cannot be pinpointed or predicted we can think only of a probability of where they might be.
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.
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.