Inside the atom history 2
In 1910 Ernest Rutherford oversaw Geiger and Marsden carrying out his famous experiment. He 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 they came out of radioactive atoms and he reckoned that they would be deflected (he had noticed their movement in air – they were sometimes displaced by large amounts). Also, unlike beta particles, all the alpha particles from a given radioactive source have the same energy.
The experiment

Figure 1. Roll over different parts to see the different results that Geiger and Marsden might have seen.
The evolution of the atomic model
Models of the atom with roll over highlights 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.
Rutherford’s 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, Bohr’s 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.
Animation of what Geiger and Marsden might have seen
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
Rutherford’s 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
(rollover highlights)
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 structure of the atom
M 1
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
Question H2

a) How did Rutherford know that the nucleus was so small?

b) How did Rutherford know that the nucleus contained most of the mass of the atom

c) Why couldn't Bohr's model of the atom work?

d) What is the name of the current theory of the atom?

The story so far
  • Dalton proposed that matter was made of tiny fundamental particles he called atoms
  • J J Thomson discovered that the electron was a part of the atom and is easily removed
  • We cannot ever see inside an atom because it is smaller than the wavelength of visible light