Rutherford's gold foil experiment or alpha scattering experiment as it is often referred to follows on from J.J. Thomson's discovery of the electron and the plum pudding model of the atom in which he proposed that the atom consisted of a sphere of positive charge in which were embedded the negatively charged electrons. 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 the 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.
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 his ideas proved that the plum pudding model proposed earlier by J.J. Thomson was in fact wrong. An outline and details of Rutherford's gold foil experiment is described below.
Rutherford's gold foil experiment
A sample of radioactive
radium metal was placed inside a lead block with a hole at one end. Radioactive radium metal has an unstable nucleus and it emits alpha particles.
Alpha particles are large heavy fast moving particles and they have lots of kinetic energy, they are like small fast moving "bullets". However they will collide with the gas molecules present in the air and so they can only travel only a few centimetres through the air.
Alpha particles have a charge of 2+, since they
consist
of 2 protons and 2 neutrons, in fact 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 proposed by Rutherford 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 radioactive radium atoms travelled towards a very thin sheet of
gold foil as outlined in the image below:
Now based on Thomson's plum pudding model of the atom Rutherford knew that the sphere of positive charge proposed by Thomson's model of the atom would not be able to stop the dense fast moving alpha particles so Rutherford expected most of the alpha particles which were fired at the thin sheet of gold foil to simply pass straight through.
Rutherford's gold foil experimental set-up
The image below outlines the set-up Rutherford used in his experiment. 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!
Rutherford discovers the nucleus
According to Thomson'splum 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 small dense bullet like projectile such as
a heavy positively chargedalpha particle. So Rutherford would have expected nearly all the
alpha particles
to pass straight through the gold foil while some may 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 indeed pass straight through the gold foil as Rutherford expected 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 of the atom. 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
Since most
of the alpha particles passed straight through the
gold foil (as shown opposite) and only a few were deflected through large angles,
Rutherford concluded that the atom consisted of mostly empty space but with a tiny centre 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 chargednucleus the greater the repulsion they would feel and the larger the angle through
which they would have been deflected through (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 of them actually 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'sgold foil experiment proved beyond any doubt that
Thomson'sPlum Pudding model
of the atom was wrong. However it still left some questions unanswered. The main problem with Rutherford's model were 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
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 much like the planets orbiting the Sun. 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.
Bohr's electron energy levels or electron shell
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.
Rutherford's gold foil experiment self-check
Fill in the missing words in the paragraph below by selecting the correct word from the drop-down menu. All the words in the drop-down menus are shown in the yellow boxes below.
gold
plum pudding
large
nucleus
alpha particles
straight
empty
bounced
Rutherford's gold foil experiment was set to test Thomson's __________ model of the atom. In Rutherford's gold foil experiment
__________ were fired at a thin __________ foil; Rutherford expected most of the alpha particles to pass __________ through the gold foil with a few experiencing minor deflections. However he was amazed by the results, while most of the alpha particles did pass straight through the gold foil some were deflected at
__________ angles and a few even __________ back. These unexpected results led Rutherford to conclude that the atom consists of mostly __________ space with a tiny, dense, positively charged __________ at its centre.
James Chadwick discovers the neutron
Note details of Chadwick's experiments are not required at GCSE. They are
mentioned here purely for your 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.
Click on the flashcards below for a quick review of the work above
What are alpha particles?
Alpha particles consist of 2 protons and 2 neutrons. They have a charge of 2+.
What did Rutherford discover following his alpha scattering experiment?
That atoms contain a small dense centre which has a positive charge. That is he discovered the nucleus.
Who suggested that the electrons orbit the nucleus in discrete energy levels?
Niels Bohr
What did James Chadwick discover
Chadwick discovered the neutron.
Key Points
Summary of Rutherford'sgold foil experiment:
Since most of the alpha particles passed straight through the
gold atoms in the foil 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 particles in the nucleus.
In Rutherford's model of the atom the electrons are simply spinning around the nucleus.
One of Professor 
Chadwick realised that when the