In the 50 years leading up to 1897, the chemical and physical properties of the atom were not fully

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In the 50 years leading up to 1897, the chemical
and physical properties of the atom were not
fully understood. Scientists at the the time
thought that the atom was a solid indestructible,
and indivisible structure. But, in 1897, when
Joseph John( known as J.J.) Thomson discovered
the electron, the entire atomic model had to be
rethought!!
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1856-1940
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J.J. Thompson (1856-1940)The electrons were
discovered in 1897. By the British physicist Sir
Thompson. Like so many great scientists, he was a
bit of a brainiac, and went to Cambridge on a
scholarship in 1876, and was elected to the
head-honchoship of the Cavendish Laboratory in
1884.
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It was a surprising decision at the time, but the
right one he was better known as a mathematician
than a physicist, but proved to be perfect for
the job. In fact he only got the job after Lord
Kelvin turned it down. In this he was in good
company his two predecessors at the job, James
Clerk Maxwell and Lord Rayleigh, also got the job
after Kelvin declined. As well as his own work,
he was instrumental in bringing a whole bunch of
guys and gals -- including Ernest Rutherford --
to Cavendish.In 1906, Thomson received the Nobel
prize for his discovery of the electron, became
Sir Thomson in 1908 and was made head of the
Royal Society in 1915. He also discovered the
proton!
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Sir Thomson suggested that atoms have parts. He
advanced the idea that cathode rays are really
streams of very small pieces of atoms. First, in
a variation of an 1895 experiment by Jean Perrin,
Thomson built a cathode ray tube ending in a pair
of metal cylinders with a slit in them. These
cylinders were in turn connected to an
electrometer, a device for catching and measuring
electrical charge.
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Perrin had found that cathode rays deposited an
electric charge. Thomson wanted to see if, by
bending the rays with a magnet, he could separate
the charge from the rays. He found that when the
rays entered the slit in the cylinders, the
electrometer measured a large amount of negative
charge. The electrometer did not register much
electric charge if the rays were bent so they
would not enter the slit. As Thomson saw it, the
negative charge and the cathode rays must somehow
be stuck together you cannot separate the charge
from the rays.
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Now Thomson thought of a new approach. A charged
particle will normally curve as it moves through
an electric field, but not if it is surrounded by
a conductor. Thomson suspected that the traces of
gas remaining in the tube were being turned into
an electrical conductor by the cathode rays
themselves. To test this idea, he took great
pains to extract nearly all of the gas from a
tube, and found that now the cathode rays did
bend in an electric field after all.
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Thomson concluded from these two experiments
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Thomsons next experiment sought to determine the
basic properties of the particles. Although he
couldn't measure directly the mass or the
electric charge of such a particle, he could
measure how much the rays were bent by a magnetic
field, and how much energy they carried. From
this data he could calculate the ratio of the
mass of a particle to its electric charge (m/e).
He collected data using a variety of tubes and
using different gases.
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By deflecting cathode rays (beams of electrons)
with electric and magnetic fields, J.J. Thomson
was able to determine the charge-to-mass ratio
for an electron.
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The results were astounding. Just as Emil
Wiechert had reported earlier that year, the
mass-to-charge ratio for cathode rays turned out
to be over one thousand times smaller than that
of a charged hydrogen atom. Either the cathode
rays carried an enormous charge (as compared with
a charged atom), or else they were amazingly
light relative to their charge.
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Thomson boldly announced the hypothesis that "we
have in the cathode rays matter in a new state, a
state in which the subdivision of matter is
carried very much further than in the ordinary
gaseous state a state in which all matter... is
of one and the same kind this matter being the
substance from which all the chemical elements
are built up."
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"Could anything at first sight seem more
impractical than a body which is so small that
its mass is an insignificant fraction of the mass
of an atom of hydrogen? --which itself is so
small that a crowd of these atoms equal in number
to the population of the whole world would be too
small to have been detected by any means then
known to science."
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In conclusion Thomson demonstrated in 1897 that
cathode rays were actually units of electrical
current made up of negatively charged particles
of subatomic size. He believed them to be an
important part of all matter and theorized a
model of atomic structure in which a quantity of
negatively charged electrons was embedded in a
circle of positive electricity, the two charges
neutralizing each other.
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