Higher and foundation tiers

The plum pudding model of the atom

In 1904 Thompson suggested a model of the atom based on his ideas and work. Thompson's model of the atom is often called the plum pudding model. His idea of what an atom looks like is a sphere of positive charge in which are embedded the electrons. A bit like a plum pudding or chocolate chip cookie. Thompson's calculations had shown that the electron was 1/1840th the mass of a hydrogen atom.

One of Thompson's students at Cambridge University, was the brilliant scientist Ernest Ruthford. In 1911 Rutherford with help from two other research scientists, Hans Geiger and Ernest Marsden conducted a now very famous experiment which lead to the idea that atoms contain a nucleus. An outline of the experiment is shown in the diagram below.

A sample of radioactive radium metal was placed inside a lead block with a hole at one end. Radium emits alpha particles. Alpha particles are large heavy slow moving particles with a charge of 2+, they consist of 2 protons and 2 neutrons, similar to the nucleus of a helium atom. Alpha particles are only able to travel a few centimetres in air so the whole experiment was carried out in a sealed container in which the air was sucked out by a vacuum pump. Next the alpha particles which were emitted by the radium travelled towards a very thin sheet of gold foil.

Rutherford was able to detect the alpha particles as they emitted a glow when they struck the fluorescent screen shown in the image below. From the image you can see that some of the alpha particles travelled straight through the gold foil but some were deflected through small angles and some even bounced stright back, as if they had hit something solid and heavy!

Rutherford discovers the nucleus

According to Thompson's plum pudding model of the atom the positive charge of the pudding was spread out over the whole atom and it would not have been able to stop a heavy bullet like projectile such as a heavy positively charged alpha particle. So Rutherford would have expected all the alpha particles to pass straight through the gold foil. Indeed most of the alpha particles did but some were deflected and some bounced straight back. This was not what Rutherford was expecting at all. This should have been impossible based on the plum pudding model!

The nuclear atom

Since most of the alpha particles passed straight through and only a few were deflected through large angles, it was concluded that the atom consisted of mostly empty space but with a tiny area positive of charge located at the centre - the nucleus! The alpha particles that were deflected were those that had come close to the nucleus , since the alpha particles have a positive charge the closer they got to the positively charged nucleus the greater the angle through which they would have been deflected (this is shown in the image opposite).

The fact that very few alpha particles, about 1 in 10 0000 were deflected straight back means that the chance of any hitting the nucleus must have been small, so the nucleus must be tiny. Rutherford was also able to calculate the size of the nucleus from his experimental work, he calculated that the nucleus was about 1/10 000th the size of the atom.

The next step

The results from Rutherford's gold foil experiment proved beyond any doubt that Thompson's Plum Pudding model was wrong. However it still left some questions unanswered. The main problem were the electrons. In Rutherford's model the electrons were simply spinning around the nucleus. Since they had a negative charge and the protons inside the nucleus had a positive charge, what was there to stop the electrons crashing into the nucleus? The answer to this problem was provided by Neils Bohr and led to the idea of electron shells or energy levels in atoms.

The Bohr atom

Following Rutherford's work we have a small dense nucleus which contains protons surrounded by a dense cloud of electrons. There were ideas that the internal structure of an atom could resemble that of the solar system, where we have planets orbiting the Sun, could a similar model exist for atoms, where we could have electrons orbiting the nucleus. Niels Bohr, a Danish physicist, suggested that the electrons orbit the nucleus in discrete well defined orbits at set distances from the nucleus. This is similar to the solar system model but different in one crucial way. The electrons are only allowed to orbit the nucleus in certain shells or energy levels. What this means in practice is that an electron could be in energy level 1 or electron shell 1 or in electron shell 2 but it CANNOT be an energy level somewhere between energy level 1 and energy level 2. A physicist might say that the energy levels are quantized, meaning only certain values are allowed. The first electron shell is lowest in energy and the second electron shell is higher in energy than shell 1 and so on..... electrons occupy the lowest energy level or shell possible at any given time.

James Chadwick discovers the neutron

Note details of Chadwick's experiments are not required at GCSE. They are mentioned purely for reader interest.
In 1932 James Chadwick discovered the neutron. His discovery followed work from a number of scientists who had noticed that when alpha particles struck certain materials an unusual type of radiation was emitted. Polonium is a highly radioactive element which emits alpha particles. It had been noted that if certain elements e.g. Beryllium where placed in the path of alpha particles a strange unknown radiation was produced, see diagram below. Chadwick realised that when the alpha particles from the polonium hit the beryllium the unknown radiation which was produced was uncharged but deeply penetrating. It was not picked up by the charged particle detector. Paraffin wax when struck with this unknown new radiation emitted protons. The protons then are recorded by the detector. Chadwick's measurements and calculations enable him to realise that this unknown radiation was in fact neutrons.

Key Points

Summary of Rutherford's gold foil experiment:

• Since most of the alpha particles passed straight through the gold atoms, then atoms are mostly empty space.
• Most of the mass of the atom is concentrated in a small positively charged core- the nucleus.
• Atoms do not have a charge, so the number of negatively charged electrons is the same as the number of positively charged protons in the nucleus.
• In Rutherford's model the electrons are simply spinning aroung the nucleus.

Check your understanding - Rutherford's gold foil experiment questions.

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