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A Brief History of the

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					A Brief History of the
  Atomic Theory
                Democritus
• 400 B.C. Greek
  philosopher
• Coined the term
  “atomos” which means,
  Indivisible.
• All matter is made of
  atoms.
• Atoms are hard, solid
  particles, made of the
  same material but are
  of different shapes and
  sizes.
                John Dalton
• 1803 English Chemist
• Atoms are solid,
  neutral spheres.
• Atoms of same
  element are the same.
• Atoms of different
  elements are different.
• Compounds form from
  the joining of atoms of
  two or more elements.
                  J.J. Thomson
• 1897 English Chemist.
• Atoms are made of even
  smaller particles.
• Called the Plum Pudding
  Model (Chocolate Chip
  Cookie Dough Model)
• Positively charged material
  through which negative
  particles are scattered.
• Since atoms are neutral,
  therefore, there must be (+)
                                 Cathode Ray Experiments;
  particles too, but Thomson     discovered the electron.
  never found them.
            Ernest Rutherford
• 1911 British physicist
• Gold Foil experiment.
• Atom has a small,
  dense positively
  charged center called
  the Nucleus.
• Negative electrons are
  scattered outside the
  nucleus.
• Most of the atom is
                            If an atom was the size of a
  empty space
                           baseball stadium, the nucleus
                           would be the size of a marble.
Rutherford’s
   Gold Foil Experiment
•A beam of + particles (alpha particles) shot
through a thin sheet of gold foil.
•Most particles passed straight through.
(Most of atom is empty space.)
•A few were deflected. (Positive core-similar
charges repel each other.)
•Very few bounced off. (Solid core is very
small.)
                   Neils Bohr
• 1913 Danish Scientist
• Planetary model.
• Electrons are held in
  place by the attraction
  between them and the +
  charged nucleus.
• Each electron occupies
  a specific energy level
  and orbit the nucleus
  like planets circling the
  sun.                        Labeled each energy level
                              by a quantum number.
 Wave Model
• Electrons are not
  discreet particles
  moving in discreet
  orbits.
• The probable location of
  an electron depends on
  how much energy it
  has.
• Electrons seem to be
  everywhere at once,
  like the moving blades
  of a fan.
        Electron Cloud Model
• Positively charged
  protons and neutral
  neutrons are held
  together with a huge
  amount of energy
  forming the nucleus of
  the atom.
• Negatively charged
  electrons move rapidly
  around the outside of
  the nucleus forming
  “clouds” of negative
  charge.
• Most of the mass of the
  atom is in the nucleus.   Quantum Model

				
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posted:10/27/2011
language:English
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