Atomic Theory

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Atomic Theory

• Early Understandings of the Structure of the Atom

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Take a minute

• Describe something you have never seen before.

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The Structure of the Atom

• What is a model?
• Model A small object, usually built to scale,
  that represents in detail another, often larger
  object.
• Scientific Model - A schematic description of a
  system, theory, or phenomenon that accounts for
  its known or inferred properties and may be used
  for further study of its characteristics.
  Scientific models are developed when a
  scientists creativity and insight are combined
  with data and observations about many similar
  scenarios.
• References http//www.answers.com/topic/model
• http//www.learner.org/channel/courses/essen
  tial/physicalsci/session2/closer1.html

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The notion of an atom

• First proposed by philosopher Democritus ( 400
  BC)

• Democritus believed that atoms were indivisible
  and indestructible.
• Democrituss ideas were limited because they
  didnt explain chemical behavior and they lacked
  experimental support.

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The Structure of the Atom

• Two discoveries led to the rebirth of the idea
  of the atom
• Lavoisier (1777) Law of Conservation of Matter
• Proust (1799) Law of Constant Composition
• Table Salt (NaCl) is always composed of 39 Na
  and 61 Cl by mass. 11 chemical ratio
• This jump-started the atom discussion again with
  a basis in experimentation rather than philosophy.

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John Daltons Atomic Theory (1808)
Elements are composed of atoms. All atoms of a
given element are identical, indivisible and
indestructible. The atoms of one element are
different from the atoms of all other elements
Compounds are composed of atoms of more than one
element. In any compound, the ratio of atoms is
a whole number or simple fraction.
Law of conservation of mass A chemical reaction
involves only the rearrangement of atoms it does
not result in their creation or destruction.
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• What modern aspects of the atom does this theory
  lack?

-Subatomic particles!!
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Experiments by J.J. Thomson (1897)

• Experiments by J.J. Thomson led to the notion of
  subatomic particles. He showed that atoms of any
  element contain negative particles.
• He worked with a cathode ray tube (CRT).
• Watch cathode ray tube simulation

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Cathode Ray Tube (CRT)

• Electricity flows from the cathode to the anode
• Cathode negatively charged plate
• Anode positively charged plate
• Thomson found that when he turned on the electric
  potential (connected the battery) an invisible
  beam was created.
• He placed a pair of charged plates around the
  CRT. The invisible beam was deflected away from
  the negatively charged plate and towards the
  positively charged plate.
• J.J. Thomson devised a way to calculate the mass
  to charge ratio of the negatively charged
  particles. He was able to prove that, no matter
  what metal was used at the cathode, a negative
  particle with the same mass to charge ratio was
  emitted.

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Millikans oil drop experiment determined
electron mass (1932 Nobel Prize in physics)
e-
charge -1.60 x 10-19 C Thomsons charge/mass of
e- -1.76 x 108 C/g
e- mass 9.10 x 10-28 g Watch
oil drop experiment animation
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This new evidence of subatomic particles led to a
new scientific model

• Plum Pudding Model of the atom.

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Nuts and Bolts of the Plum Pudding Model

• Atoms consist of
• 1. A spherical cloud of positive charge.
• 2. Embedded within the cloud are negatively
  charged electrons.
• - You can liken this model to a chocolate chip
  cookie ball. The dough is the cloud of positive
  charge, and the chocolate chips are the embedded
  electrons.

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Experiments by Ernest Rutherford (1909)

• Rutherford worked in J.J. Thomsons lab. He did
  intense work concerning alpha (a) particles
  (positively charged particles).

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What would happen to the alpha particles if the
Plum Pudding model was correct?

• Knowing what you know about attractions between
  particles with opposite charges and particles
  with like charges make a prediction

The alpha particles would travel right through
the cloud of positive charge. But, some would be
deflected slightly by the electrons.
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Actual Results
Watch Rutherford experiment animation (alpha
particle scattering)
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Rutherfords Gold Foil
Positively charged alpha particles are deflected
by the nucleus
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Nuts and Bolts of Rutherfords Model

• An atom contains a dense positively charged
  nucleus
• The particles in the nucleus that are positively
  charged are called protons.
• An atom contains negatively charged particles
  called electrons that move around the nucleus
  like bees in a hive.
• The rest of the atom is mostly empty space

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One more subatomic particle

• In 1932, James Chadwick, a coworker of
  Rutherford, was able to show that a nucleus also
  contained a third subatomic particle, the
  neutron.

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Chadwick Model of the Atom

• An atom contains a dense nucleus made of charged
  protons and neutral neutrons.
• The negative electrons have a much smaller mass
  and are located outside of the nucleus.
• The rest of the atom is mostly empty space.

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