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The atom theory of matter
Electrons and nuclei
The discovery of the electron
Thomson (1897)
If we apply an electric tension
between the end-sides of the bulb
Crookes discovered “cathode rays”
(1879)
In 1897 Thomson discovered that cathode rays
are made up of the same corpuscles
He built an apparatus provided with deflecting
electric and magnetic field (such as the one
that we saw in our lab last year), and he
succeeded in determining the value of the
ratio charge-to-mass, the so called specific
charge of what he named “electron”.
e
11
 1,76 10 C / kg
m
Electron is a very light corpuscle
Most of the mass of an atom should be made of
positive matter.
Thomson proposed the plum pudding model:
The atom is a sort of “plum pudding” with
electron embedded (like the plums) in a
heavily positive dough.
the plum pudding model
The electrons are the blue points, the red sphere is
the positive dough
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Thomson’s model was decisively repudiated by
Rutherford’s scattering experiment
Rutherford tested the Thomson model by firing
alfa-particles into a sheet of gold foil …
He found that it doesn’t work well!
Most of the alfa-particles passed right trough,
but a few bounced right back, indicating that
they had hit something small and heavy.
What are alfa-rays?
Let’s see X-rays, first!
The luminescence that we can see when cathode rays
hit the glass or the sheet of ceramic foil is due to
the rapid deceleration that the electrons suffer.
The electromagnetic energy irradiated can be up to 3
keV (this is the energy that the electronic gun
supplies the electrons).
If the energy has increased up to 30 keV than the
electromagnetic energy is irradiated in the X-ray
range of the electromagnetic spectrum (these rays
are also invisible and mysterious, so Roentgen, who
first detected them in 1895, called them X-rays)
Bequerel discovered ,, rays
(1896)
Now …
Now we know that -rays are a flow of helium
nuclei (i.e. two protons and two neutrons), rays are a flow of electrons, and -rays are
electromagnetic waves (they are actually a
flow of photons, as we’ll see later)
The experimental data suggested a
planetary model.
The Rutherford experiment and his
model of an atom
• Electrons orbit around a circle whose radius is about
10-10 m . At the centre of the circle there is the
nucleus, a small sphere whose radius is about 10-15 m.
• This model permits us to understand why most of the
alfa-particles pass right trough the atom: there is a lot
of empty space there! Instead, the unlikely particles
which move just towards the nucleus, suffer a very
strong repulsion: they can be bounced right back!