Alpha-Particle
Scattering Experiments and
Rutherford's Nuclear Atom
Identification of the nucleus of the atom by Rutherford in 1911
is one of the greatest landmarks in science. It was followed by
the recognition of the proton and neutron as constituents of nucleus.
But before we look at the [scattering] experiments and Rutherford's atomic model, it is
worth noting the neglected publication of the Janpanese physicist,
Nagaoka, in 1904, to which Rutherford himself drew attention.
Maybe the atom is
just like Saturn...
In 1904, Nagaoka published his tentative theory of the stable
nuclear atom, with a small, massive, positively charged nucleus
around which circled a large number of negatively charged [oscillating]
electrons. He compared the atom to Saturn, the planet with "rings"
around it. He proposed that the atom could be approximately visualized
by replacing Saturn's satellites (i.e. Saturn's rings) with electrons
and substituting the attracting centre by a positively charged
particle. He concluded his paper by suggesting that it "may
serve as a hint to a more complete solutions of atomic structure"
The Alpha-Particle Scattering
Experiments
"...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 15-inch shell at a piece of tissue paper and it came
back and hit you..." said Rutherford, 1936.
In 1909, Geiger and Marsden , at Rutherford's suggestion, investigated the possibility of larger angle scattering of alpha particles by matter. The experimental arrangement was shown in the figure:
They worked with the gold foils with
different thicknesses. Nearly all of the alpha particles passed
through the gold foil of about 1 µm thick while a small
number were deflected and a few even bounced back.
They also estimated in every 8000 alpha particles bombarding the
foil, one alpha particle would be bounced back, under the conditions
of their experiment Rutherford later expressed that it seemed
rather like firing a machine gun at tissue paper, and finding
that some of the bullets bounce back!
What's wrong with the
bun?
| As the Thomson atom was considered to consist of negatively charged electrons embedded in a sphere of positive charge, there was no region within the atom where there would be an appreciable net concentration of charge. Therefore, there was no strong electric field to deflect the alpha particle by over 90°. The Thomson atom was in complete disagreement with the observation obtained in the scattering experiment. |
 |
Rutherford's Theory
(1911)
To account for this very surprising result, Rutherford suggested
in 1911 that all the positive charge and nearly all the mass were
concentrated in a very tiny region called the nucleus, at the
centre of the atom. The rest of the atom was largely empty space
through which the alpha particles could travel straight on.
Rutherford considered the electrons to be outside the nucleus.
The electrostatic attraction between the two opposite charges
( the negatively charged electrons and the positively charged
nucleus) provided the [centripetal
force] for the circular movement of electrons
around the nucleus. Since they were relatively far away from the
nucleus, their negative charge did not act as a shield to hinder
the penetration of the alpha particles through the atom.
 |
The scattering of alpha particles was due to the strong [electrostatic repulsion]which the alpha particles experienced when they approached the positive nucleus. The closer the approach, the greater the scattering. |
The scattering analogue
The diagram shows a gravitational analogue of the scattering experiment. A ball-bearing rolls down a ramp to climb up a 'hill'. The ball-bearing is deflected and the deflection depends on its direction of approach.
Experimental Tests
of Rutherford's Theory
Rutherford derived an expression for the number of alpha particles
deflected through various angles. Following Rutherford's analysis,
Geiger and Marsden undertook a thorough investigation of Rutherford's
alpha-scattering formula and published their results in 1913.
The apparatus they used were shown in the figure:
A fine beam of alpha particles from a radioactive source fell
on a thin foil of gold, platinum or other metal in an [evacuated]
box. The angular deflection of the particles was measured by using
a microscope to observe the scintillations on the zinc-sulphide
screen. The screen and microscope could be rotated together relative
to the foil and the source.
After spending many hours in a darkened room, counting the scintillations
for a wide range of angles, they obtained results which gave approximate
agreement with the prediction of Rutherford's scattering formula.
Size of the nucleus
Geiger and Marsden's investigations of the scattering of alpha
particles by metal foils gave the first indication of the size
of the nucleus. It is now known that the nuclear radii are in
the range from 1 x 10-15 to 8 x 10-15 m.
If we were to scale up the whole atom to the size of Hong Kong,
then the nucleus would be about the size of a man. The electrons
would be like flies.
Rutherford pointed out that he had not given consideration to
the stability of the atom. However, in 1913, another scientist,
Niels Bohr extended Rutherford's Theory and arrived at a more
accurate atomic structure.
(C)1999 ThinkQuest Team 27954