The Atomic Model

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The Atomic Model

• Chem 9

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Atomic Model

• Atoms are so small that, even today, direct
  visual inspection is all but impossible. Our
  model of the atom is based on indirect
  experimental data. Because of this, our model of
  the atom changes as our experimental ability
  improves

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The Greek Model

• Democritus - A Greek Philosopher around the year
  400 BC.
• Democritus concluded that matter could not be
  divided into smaller and smaller pieces forever.
• Eventually, the smallest piece of matter would be
  found. He used the word "Atomos" to describe the
  smallest possible piece of matter.

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The Dalton Model Billiard Ball Model

• John Dalton - The English chemist that proposed
• first Atomic Theory in 1803.
• All elements are composed of indivisible
  particles.
• Atoms of the same element are exactly alike.
• Atoms of different elements are different.
• Compounds are formed by joining atoms of two or
  more elements

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What did Dalton base his theory on?

• Law of Conservation of Mass (Antoine Lavosier,
  1789)
• Atoms are neither created or destroyed (under
  normal chemical reactions)
• Law of Definite Proportion, (Proust's Law)
• Atom ratio is fixed, so mass must be constant.
• Law of Multiple Proportions (Dalton)
• Applies where two elements, A and B, form more
  than one compound.

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How Big is an Atom

• Imagine a penny, made up of copper (Cu)
• If we ground up the penny into dust, the dust
  would still have the same properties as copper

If you placed atoms of copper in a row, 100 000
000 atoms would make a line 1 centimeter long
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How Big is an Atom?

• A penny contains around 2.4 x 1022 atoms of
  copper
• Can we see atoms?
• To see atoms we need a scanning tunneling
  electron microscope or an ion beam microscope

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• A micrograph of a Nickel-Molybdenum (Ni4Mo)
  bi-metallic compound, each dot is a single atom
• The specimen is in the form of a needle that is
  over 1000 times sharper than an ordinary
  household sewing needle. In fact, the end of the
  needle is so sharp that it cannot be seen by the
  naked eye or even a standard optical microscope

http//www.ornl.gov
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Whats Wrong with DAT?

• Most of Daltons theory is accepted today
• However, We now know that atoms ARE divisible
• Atoms contain sub-atomic particles
• ELECTRONS
• PROTONS
• NEUTRONS

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The Thomson Model Chocolate Chip Cookie Model

• J. J. Thomson 1856 1940 - The English scientist
  who discovered Electrons in 1887.
• Thomson determined the charge of an electron to
  be negative in his cathode ray experiment
• Thompson initially called electrons CATHODE RAYS

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Electrons
Positive
Negative
Vacuum
High Voltage
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Robert A. Millikan (1868 1953)

• 1900 Electron defined as a small charged
  particle approximately 1/2000 the mass of a
  Hydrogen atom
• 1916 -determined the charge carried by an
  electron
• OIL DROP EXPERIMENT

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Protons E. Goldstein 1850 -1930

• 1866 Eugene Goldstein found evidence of
  positively charged particles traveling in the
  opposite direction to electrons in a cathode ray
  tube
• He named them CANAL RAYS
• He deduced that canal rays were positively
  charged particles we now know as PROTONS

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Neutron

• Sir James Chadwick 1891 - 1974
• Discovered
• the neutron in 1932
• Has no charge but a mass close to that of the
  proton

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The Rutherford Model Gold Foil Experiment

• Ernest Rutherford - The British physicist who, in
  1908, proved the atom had a small, dense,
  positively charged Nucleus.
• Rutherford's model proposed that an atom is
  mostly empty space. There is a small, positive
  nucleus with the negative electrons scattered
  around the outside edge.

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Vacuum
Flourescent Screen
Au Foil
a-source
c. Rutherford Appleton Laboratories
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Gold Foil Experiment
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Conclusions

• Rutherford concluded that the atom is mostly
  empty space
• However, the atom contains a nucleus where the
  mass and positive charge of the atom are
  concentrated
• We now know that the nucleus contains PROTONS and
  NEUTRONS
• Electrons ORBIT the nucleus but are not part of it

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The Bohr Model The Planet Model

• Niels Bohr - The Danish scientist who, in 1913,
  proposed the Planetary Model of the atom.
• Electrons move in definite orbits around the
  nucleus, like planets moving around the nucleus.
  Bohr proposed that each electron moves in a
  specific energy level.

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Properties of Atoms
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The Wave Model

• Based on Wave Mechanics, this model proposes that
  electrons have NO definite path in an atom. The
  probable location of an electron is based on how
  much energy it has. The more energy an electron
  has, the farther from the nucleus. The small,
  positively charged nucleus is surrounded by a
  large space in which there are enough electrons
  to make the atom neutral.

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Backdrops - These are full sized backdrops, just
scale them up! - Can be Copy-Pasted out of
Templates for use anywhere!
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