The History of the Atom

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The History of the Atom
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Democritus(c.460-c.370 BCE)

• Democritus was a philosopher in ancient Greece.
• He thought that all matter was made of tiny
  particles that could not be divided.
• The Greek word atomos means indivisible.

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John Dalton(1766-1844)

• Dalton was an English scientist.
• He developed modern atomic theory.
• His model of the atom is sometimes called the
  billiard-ball model.

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Daltons Billiard-Ball Model

• All matter is made of tiny particles called
  atoms.
• Atoms of the same element are identical.
• Atoms can combine to form compounds.
• Chemical reactions change the grouping of atoms,
  but not the atoms themselves.

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Sir J. J. Thomson(1856-1940)

• Thomson was a British physicist.
• He did experiments on cathode rays and discovered
  the electron.
• In 1906, he was awarded a Nobel prize for his
  discovery.
• His model of the atom is called the plum-pudding
  model.

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Thomsons Plum-Pudding Model

• Dalton realised that negatively charged electrons
  could come from an atom.
• He proposed the plum-pudding model of the atom,
  suggesting that atoms consist of
  negatively-charged electrons in a sea of
  positive charge.

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Ernest Rutherford(1871-1937)

• Rutherford was a chemist from Nelson, New
  Zealand.
• Based on the results of his gold-foil experiment,
  he proposed that most of the mass of an atom is
  concentrated in a central nucleus.
• He won the Nobel Prize in Chemistry in 1908 for
  his work.

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Rutherfords Gold-Foil Experiment

• In his famous gold-foil experiment, Rutherford
  fired alpha particles at a thin sheet of gold
  foil.
• He found that some particles were deflected
  through large angles, and some even bounced back.
  

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Rutherfords Nuclear Model

• Rutherford concluded that
• most of the mass in an atom must be in a very
  small, positively-charged nucleus in the centre
  of the atom
• electrons spin around this central nucleus
• there was a basic unit of positive charge in the
  nucleus, called the proton.

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• Niels Bohr (1885-1962) realised that the
  electrons could only occupy fixed orbits around
  the nucleus.

• Louis de Broglie (1892-1987) proposed that
  electrons can be regarded as waves, resulting in
  an electron cloud around the nucleus.
• Sir James Chadwick (1891-1974) discovered the
  neutron, a nuclear particle with similar mass to
  a proton but no electrical charge.

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References

• Atom Image http//www.turbosquid.com/FullPreview/
  Index.cfm/ID/197928
• Democritus http//smccd.net/accounts/goth/MainPag
  es/Chron/Democritus.jpeg
• Dalton http//en.wikipedia.org/wiki/John_Dalton
• Thomson http//en.wikipedia.org/wiki/J_J_Thomson
• Plum-Pudding Atom https//reich-chemistry.wikispa
  ces.com/file/view/348px-Plum_pudding_atom_svg.png
• Ernest Rutherford http//en.wikipedia.org/wiki/Ea
  rnest_Rutherford http//nobelprize.org/nobel_prize
  s/chemistry/laureates/1908/rutherford-bio.html
• Gold-Foil Experiment http//wps.prenhall.com/wps/
  media/objects/602/616516/Media_Assets/Chapter02/Te
  xt_Images/FG02_05.JPG
• Nuclear Atom http//upload.wikimedia.org/wikipedi
  a/commons/thumb/e/e1/Stylised_Lithium_Atom.svg/180
  px-Stylised_Lithium_Atom.svg.png
• Electron Cloud http//www.csmate.colostate.edu/cl
  tw/cohortpages/viney_old1/atom.jpg