Higher and foundation tiers

3d model of Thomson's Plum pudding model of the atom.

This page deals with Rutherford's gold foil experiment or alpha scattering experiment as it is often referred to as and follows on from the page which details J.J. Thomson's discovery of the electron and the plum pudding model of the atom which he proposed. If you are not familiar with this model of the atom then have a quick recap before reading this page.

The plum pudding model of the atom

In 1904 Professor J.J. Thomson suggested a model to describe the internal structure of an atom based on his ideas and experimental work. Thomson's model of the atom is often called the plum pudding model. His model described the atom as a sphere of positive charge in which are embedded the negatively charged electrons; a bit like a plum pudding or chocolate chip cookie.

portrait of Ernest Rutherford working in the physics lab One of Professor Thomson's students at Cambridge University was the brilliant scientist Ernest Rutherford. In 1911 Rutherford with help from two other research scientists; Hans Geiger and Ernest Marsden conducted a now very famous experiment which led to the idea that atoms contain a nucleus and proved that the plum pudding model proposed earlier by J.J. Thomson was wrong. An outline and details of Rutherford's gold foil experiment is described below.

Rutherford's gold foil experiment

3d model of alpha particles.

A sample of radioactive radium metal was placed inside a lead block with a hole at one end. Radioactive radium metal emits alpha particles. Alpha particles are large heavy fast moving particles (they travel about 5% light speed in air) and they have lots of kinetic energy, they are like small "bullets". However they can only travel only a few centimetres in air. Alpha particles have a charge of 2+, since they consist of 2 protons and 2 neutrons. Alpha particles are similar to the nucleus of a helium atom. Since alpha particles are only able to travel a few centimetres in air the whole experiment was carried out in a sealed container in which the most of 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 expected most of the alpha particles which were fired at the gold foil to simply pass straight through. The set-up Rutherford used is shown below. He 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 most of the alpha particles travelled straight through the gold foil but some were deflected through small angles and a few even bounced straight back; as if they had hit something solid and heavy!

Model showing the set-up used in Rutherford's gold foil experiment. Rutherford was amazed when some of the alpha particles actually bounced back off the gold foil, he described it by saying that it was as if someone had fired an artillery shell at a piece of tissue paper and it came back and hit you.

Rutherford discovers the nucleus

According to Thomson's plum pudding model of the atom the positive charge of the "pudding" was spread out over the whole atom in a cloud of positive charge 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 nearly all the alpha particles to pass straight through the gold foil while some would have been deflected through small angles due to the repulsion from the cloud of positive charge. The results of the experiment did show that most of the alpha particles did pass straight through the gold foil but some were deflected through large angles and some even bounced straight back as if they had hit something "solid" inside the gold atoms. This was not what Rutherford was expecting at all. This should have been impossible based on the plum pudding model! Indeed when Geiger and Marsden presented Rutherford with the results of the scattering experiment Rutherford said:

........."Then I remember two or three days later, Geiger coming to me in great excitement and saying we have been able to get some of the alpha particles coming backwards. It was quite the most incredible event that had ever happened to me in my life. It was almost as incredible as if you had fired a 15 inch artillery shell at a piece of tissue paper and it came back and hit you. On consideration I realised that this scattering backwards must be the result of a single collision and when I make my calculations I saw that it was impossible to get anything of the order of magnitude unless you took a system in which the greater mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre carrying a positive charge"........

The nuclear atom

Rutherford's scattering experiment

Since most of the alpha particles passed straight through the gold foil (as shown opposite) 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 of positive charge located at the centre of the atom- 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 and 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 Thomson's Plum Pudding model was wrong. However it still left some questions unanswered. The main problem with Rutherford's model was the electrons. In Rutherford's model of the atom we have a small dense positively charged nucleus with the electrons simply spinning around it, much like the planets in the solar system as they orbit the Sun. However since the electrons have a negative charge and the nucleus has a positive charge what was there to stop the electrons simply crashing into the nucleus? The answer to this problem was provided by Niels Bohr and led to the idea of electron shells or energy levels in atoms.

The Bohr atom

Portrait of Niels Bohr

Following Rutherford's work the model to represent the internal structure of the atom that was suggested was one where a small dense nucleus was surrounded by a dense cloud of electrons which orbited around the newly discovered nucleus. Some scientists had the idea that the internal structure of an atom could resemble that of the solar system with the electrons orbiting the small dense central 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's expt to discovery the nucleus 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:

Practice questions

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

Check your understanding - Quick Quiz on Rutherford's gold foil experiment.