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Sir Joseph John "J
...
" Thomson, OM, FRS (18 December 1856 – 30 August 1940)
J
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He is credited
for the discovery of the electron and of isotopes, and the invention of
the mass spectrometer
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Discovery of the electron
Until 1897, scientists believed atoms were indivisible, the ultimate
particles of matter, but Thomson proved them wrong when he
discovered that atoms contained particles known as electrons
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Thomson found that the rays could be deflected by an electric field
(in addition to magnetic fields, which was already known)
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This showed that cathode rays were matter, but he found that the particles were
about 2000 times lighter than the mass of the lightest atom, hydrogen
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(Later
scientists preferred the name electron which had been suggested by George Johnstone Stoney in
1894, prior to Thomson's actual discovery)
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He thus concluded that atoms were divisible, and that the corpuscles were their building
blocks
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Isotopes and mass spectrometry
In the bottom right corner of this photographic plate are
markings for the two isotopes of neon: neon-20 and neon-22
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Thomson observed two
patches of light on the photographic plate (see image on right),
which suggested two different parabolas of deflection
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This was the first evidence for isotopes of a stable element; Frederick Soddy had previously
proposed the existence of isotopes to explain the decay of certain radioactive elements
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W
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J
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Other work
In 1905 Thomson discovered the natural radioactivity of potassium
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Previous
theories allowed various numbers of electrons
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The aetherial hypothesis was vague, but the
particle hypothesis was definite enough for Thomson to test
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Thomson set out to investigate whether or not he could
actually separate the charge from the rays
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Thomson could trace the path of the ray by observing the
phosphorescent patch it created where it hit the surface of
the tube
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He concluded that the negative charge and the rays were one and the same
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Cathode rays were
emitted from the cathode C, passed through
slits A (the anode) and B (grounded), then through the electric field generated between plates D and
E, finally impacting the surface at the far end
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In his second experiment, he investigated whether or not the rays could be deflected by an electric
field
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Thomson constructed a Crookes tube with a near-perfect vacuum
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The rays were sharpened to a beam by two metal slits - the
first of these slits doubled as the anode, the second was connected to the earth
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The end of the tube was a large sphere where the beam
would impact on the glass, created a glowing patch
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When the upper plate was connected to the negative pole of the battery and the lower plate to the
positive pole, the glowing patch moved downwards, and when the polarity was reversed, the patch
moved upwards
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In his third experiment, Thomson measured the mass-to-charge
ratio of the cathode rays by measuring how much they were
deflected by a magnetic field and how much energy they carried
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Conclusions
As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as if
they were negatively electrified, and are acted on by a magnetic force in just the way in which this
force would act on a negatively electrified body moving along the path of these rays, I can see no
escape from the conclusion that they are charges of negative electricity carried by particles of
matter
...
J
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If, in the very intense electric field in the neighborhood of the cathode, the molecules of the gas are
dissociated and are split up, not into the ordinary chemical atoms, but into these primordial atoms,
which we shall for brevity call corpuscles; and if these corpuscles are charged with electricity and
projected from the cathode by the electric field, they would behave exactly like the cathode rays
...
J
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This model was later proved incorrect when Ernest
Rutherford showed that the positive charge is concentrated in the nucleus of the atom

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