Higher and foundation tiers
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 straight back, as if they had
hit something solid and heavy!
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!
Since most
of the alpha particles passed straight through the
gold foil (as shon 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, he calculated
that the nucleus was about 1/10 000th the size of the atom.
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 of the atom we have a small dense positively charged nucleus with the electrons simply spinning around it. Since the electrons have a negative charge and the nucleus had a positive charge, what was there to stop the electrons simply 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.
Following Rutherford's work we have a small dense
nucleus 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.
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 were in fact neutrons.
Summary of Rutherford's gold foil experiment: