1. The nuclear atom page 3
Rutherford and the nucleus

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.
Rutherford apparatus interactive graphic
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.

The experiment
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.
The findings
Animation of what Geiger and Marsden might have seen
"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
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 and passed straight through the foil
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 scale of the nucleus
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 10–15 m. 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.

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.

Models of the atom with roll over highlights Neils Bohr's proposed a 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.

Question 2
a) How could Rutherford tell that the nucleus has a large mass?
b) The diameter of the nucleus is 105 times smaller than that of the atom. How much smaller are the cross sectional area and the volume of the nucleus?
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