Atomic Theory

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Atomic Theory
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• Richard Feynman If we were able to pass along
  only a bit of scientific knowledge to future
  generations, what would it be?

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• That matter is made of atoms

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• Democritus-Smelling Bread

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Leucippus

• - Nothing happens at random , but everything
  from reason and by necessity.

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Atomic theory first originated with Greek
philosophers around 2500 years ago. This basic
theory remained unchanged until the 19th century
when it first became possible to test the theory
with more sophisticated experiments. As science
has rapidly advanced over the past few centuries
the atomic theory has been refined in accordance
with the accepted scientific principles and
theories of the time. Advances in technology
and theory that have allowed experiments to be
designed that enable us to probe matter to
microscopic scales.
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Ancient Atomic Theory
  The atomic theory of matter was first proposed
by Leucippus, a Greek philosopher who lived at
around 400BC. At this time the Greeks were trying
to understand the way matter is made. According
to Anassagora, it is possible to subdivide matter
in smaller and smaller parts, and he proposed
that this process can be continued with no limit.
In Anassagora's view, you can always divide a bit
of substance into two parts, and each of these
parts is also divisible into two parts, and so
on--no matter how small each part gets there is
no problem dividing it again into even smaller
parts. But according to Leucippus, eventually
you arrive at small particles which can not be
further subdivided. Leucippus called these
indivisible particles atoms (from the Greek word
atomos, meaning indivisible).
Leucippus
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Ancient Atomic Theory
Leucippus's atomic theory was further developed
by his disciple, Democritus who concluded that
infinite divisibility of a substance belongs only
in the imaginary world of mathematics. Democritus
suggested the atomic theory, explaining that all
things are "composed of minute, invisible,
indestructible particles of pure matter which
move about eternally in infinite empty.". If a
sample of a pure element was divided into smaller
and smaller parts, eventually a point would be
reached at which no further cutting would be
possiblethis was the atom of that element.
Democritus
According to the ancient Greeks, atoms were all
made of the same basic material, but atoms of
different elements had different sizes and
shapes. The sizes, shapes, and arrangements of a
materials atoms determined the materials
properties. It was believed that there were four
elements that all thing were mare from Earth,
Air, Fire and Water.
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The Modern Atomic Theory
For centuries scientists did not have the methods
or technology to test their theories about the
basic structure of matter, so people accepted the
ancient Greek view.
In the 19th century John Dalton made inferences
that exhibited how atoms bond together in
definite proportions. Dalton was able to say that
atoms of different elements combine in whole
number ratios.
This theory, to go along with four other
theories, made up what Dalton called the "Modern
Atomic Theory. Included in these were two
theories that stated atoms could not be divided
or destroyed, a theory that stated different
elements contain different chemical properties,
and atoms of the same element contain the same
chemical properties
John Dalton
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 Dalton's Atomic Theory

• 1) All matter is made of atoms. Atoms are
  indivisible and indestructible.
• 2) All atoms of a given element are identical in
  mass and properties
• 3) Compounds are formed by a combination of two
  or more different kinds of atoms.
• 4) A chemical reaction is a rearrangement of atoms

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The Modern Atomic Theory
Dalton made two assertions about atoms (1) atoms
of each element are all identical to one another
but different from the atoms of all other
elements, and (2) atoms of different elements can
combine to form more complex substances.
Although the two theories that speculated atoms
couldn't be divided were false, Dalton
contributed greatly to the advances of atomic
theory, and would greatly influence J.J. Thomson
in his own discoveries.
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Expanding the Modern Atomic Theory
J.J. Thomson is the person who is credited for
discovering the electron. Thomson created a tube
that had a positively charged anode on one side
and a negatively charged cathode on the other
side. Thomson then applied a magnet to the middle
of the tube and discovered that negatively
charged particles were emanating towards the
positive magnetic field. From this, Thomson
concluded that negatively charged particles,
called electrons, were present in atoms.
J.J. Thomson
Thomson then created the Plum Pudding model,
which suggested that electrons and protons were
randomly placed throughout the atom.
The Plum Pudding
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Thomsons Cathode Ray Experiment Plum Pudding
Model
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Rutherford's Experiment
In 1911 British scientist Ernest Rutherford set
out to test Thomsons proposal by firing a beam
of charged particles at atoms. Alpha particles
are heavy particles with twice the positive
charge of a proton. Alpha particles are now known
to be the nuclei of helium atoms, which contain
two protons and two neutrons. Ernest
Rutherford's experiment was to emit alpha
particles towards a thin gold sheet. Rutherford
would then determine where the deflections of the
alpha particles would go, and therefore be able
to theorize what kind of placement protons and
electrons had.
Ernest Rutherford
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Rutherford's Experiment
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Rutherford's Experiment
Rutherford observed that most of the alpha
particles went straight through the foil. However
a large proportion were deflected through small
angles and some (though very few) deflected
straight back. Rutherford then theorized that
there was something called a nucleus, which
contained a high density of positively charged
particles. Rutherford was able to say there was a
nucleus because alpha particles that deflected
right back must have hit something more massive
and with a strong positive charge. This led
Rutherford to propose a very different model for
the atom.
Instead of supposing that the positive charge and
mass were spread throughout the volume of the
atom, he theorized that it was concentrated in
the center of the atom. Rutherford called this
concentrated region of electric charge the
nucleus of the atom.
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References

• encarta.msn.com/encyclopedia
• www.funsci.com/fun3_en/democritus/democritus
• www.aare.edu.au
• www.absoluteastronomy.com/encyclopedia
• http//www.foresthills.edu/anderson/rod/p2notes/Hi
  st_atomic_theory.ppt
• http//wps.prenhall.com/wps/media/objects/602/6165
  16/Media_Assets/Chapter02/Text_Images/FG02_03.JPG
• edited by Aysun Simpson and Ayse Yavuz