The Brief History of the Atom

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The (Brief) History of the Atom

• Part 1

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?400 B.C. Democritus, Greek Philosopher
Concluded that when matter was divided,
eventually you get to indivisible particles,
called atoms (from the Greek word meaning
indivisible).
?400 B.C. Plato, Greek Philosopher
Believed that there were only 4 elements (fire,
earth, air, water) and that they were the
building blocks of all matter. Each had its own
unique shape.
?350 B.C.Aristotle, Greek Philosopher
Strongly opposed the idea of atoms. Spoke out
vigorously against Democritus and philosophers
who agreed with him.
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• Aristotle Plato held the prevailing thought for
  over 1000 years, despite a complete lack of
  evidence.
• The Greeks had no real way to test their
  theories.

Late 1700s Antoine Lavoisier the Father of
Modern Chemistry
French Chemist Called the father of Modern
Chemistry because he took the scattered
discoveries of others and began to make sense of
them. Discovered the Law of Conservation of
mass, among other things. This was important to
atomic theory because it proved that matter was
not being destroyed in chemical reactions.
interesting side note Lavoisier was sent to
the guillotine in 1793 during the French
Revolution just a few months after Marie
Antoinette lost her head.
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• ?1808 John Dalton,
• English Quaker, schoolteacher

• Developed a series of rules describing the
  behavior of atoms based on his experiments on how
  elements combined in Chemical reactions.
• All matter made up of tiny, indestructible
  particles called atoms.
• 2. All atoms of a given element are identical to
  each other (i.e. have the same physical
  chemical properties).
• 3. Atoms of different elements have different
  properties. They might be similar, but they are
  not the same.
• 4. Atoms arent created or destroyed in chemical
  reactions (they obey the Law of Conservation of
  Mass)
• 5. Atoms of different elements form compounds in
  whole number ratios.
• Later discoveries proved that these rules
  actually arent true, but this is still pretty
  good for a country school teacher!

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• 1897 J.J. Thompson English Physicist

Discovered electrons by passing an electric
current through gasses contained in glass tubing.
Learned that electrons were very tiny compared to
protons. Protons had already been discovered
in 1886 by E. Goldstein during a similar
experiment.
Thompson came up with the plum pudding model of
atoms- larger protons smaller electrons all
jumbled together, running around in a ball.
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1911 Ernest Rutherford Physicist
Rutherford was notoriously scornful of chemists-
Won the Nobel prize in Chemistry in 1908 (irony)
Discovered the nucleus of the atom. This was a
very big deal as it changed the concept of the
atomic model drastically. Suddenly, protons are
hanging out in the center of an atom, and
electrons are circling them. The rest of the atom
was mostly empty space.
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• Scientists were puzzled by several things
• The fact that electrons did not fall into the
  nucleus of that atom
• Electrons didnt run out of energy- they kept
  moving at extremely high speeds
• How did protons manage to pack into such a small
  space without blowing themselves the atom apart
  (all those charges tightly packed together)

• 1932 James Chadwick English Physicist

Discovered the neutron- it has no charge, but is
roughly the same size and mass as a proton, and
is also found in the nucleus. This explained why
the nucleus doesnt tear itself apart.
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• This basic history of the atom only covers the
  discovery of parts of the atom.
• So what does the atom look like?
• Stay Tuned To Find out!!!