Nuclear History

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Nuclear History

• Chemistry 332

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Seven Important Eras

• Pre-atomists
• Early atomists
• Late atomists
• Plum pudding model
• Nuclear model
• Planetary model
• Quantum mechanical model
• For a more complete listing check out the nuclear
  history page

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1. Pre-Atomists c. 650 BC

• What was the world made of?
• Thales- water because it was everywhere
• Anaximenes- air because it was around and over us
• Heraclitus- fire because it was an agent of
  change
• Empedocles- offered ideas of opposite pairs of
  earth/air and fire/water
• Aristotle- combined all above ideas and added the
  5th element ether

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2. Early Atomists c. 470 BC- 55BC

• Leucippus- if you cut something in half enough
  times, you will reach a particle that can no
  longer be cut

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Democritus

• Indivisible atomos
• atoms were distinct in size and shape
• each element was therefore unique
• substances of the world were made from
  combinations of different atoms
• substances could be changed by altering the
  mixture of the composition

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Atoms or Not?

• Aristotles followers said how can you have a
  piece of finite matter so small it cant be cut
  in half.
• Atoms dont make sense
• Besides- look at the proof of the 5 element
  theory

• Democritus followers said that you just cant
  keep cutting something in half.
• There must be some endpoint to the mess somewhere
  that is the basic building block of the universe

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Who won?

• Aristotles theories could be easily understood
• The four (5) elements could be easily seen and
  understood
• Democritus had no proof

Smoke air
fire
ashes earth
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A little theory goes a long way

• Because of this, the ideas of Aristotle prevailed
  scientific thinking for a long time
• How long?
• The next major theory doesnt arrive until the
  1800s!
• Aristotles ideas lasted about 2000 years!!!

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Were they Scientists?

• Were Aristotle and Democritus scientists?
• What makes a scientist?
• Performing controlled experiments
• Since neither could really did any experiments
  they were not scientists

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So what were they?

• What do you call a theorists who does not perform
  experiments?
• A Philosopher

umatoms
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3. Late Atomists

• John Dalton (1766-1844) reinvents the ideas of
  atoms
• Dalton begins to perform experiments with matter
  which chips away at Aristotles theories
• Multiple proportions- atoms can combine in
  different whole number ratios to make different
  substances
• CO -carbon monoxide is very different from
• CO2- carbon dioxide
• Dalton found that different masses of different
  elements combined in simple whole number ratios
  of masses

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So what?

• Two of Daltons contemporaries were Nicholson and
  Carlisle
• If Aristotle was right, then none of the four
  elements (earth, air, fire, or water) could be
  broken down into further elements
• Right about this time, two Italians, Volta and
  Galvani each invented an electrical stack which
  could deliver a constant electrical current
• Nicholson and Carlisle ran electricity through
  water and.
• It decomposed!!!

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What did it decompose into?

• They thought
• HO --gt O H
• But it came out in a 21 ratio of HO
• Therefore (after others experiments)
• 2 H2O --gt 2 H2 O2

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And So?

• Atoms came to be accepted in the general world
• So what was the next question?
• Whats inside atoms?

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4. Plum Pudding Model

• JJ Thomson (1856-1940)
• Was using a Crookes tube
• Found that running electricity through a gaseous
  element produced a stream of particles

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Experimented with magnets
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Experiment 2

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Conclusions?

• The stream of particles was

• Negative

• Overall, atoms are...

• Neutral

• Therefore, atoms must also contain charges that
  are...

• Positive

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Plum Pudding Model

• Thomson figured out that atoms contained positive
  and negative charges but had no idea how they
  were arranged.
• He guessed there was a diffuse positive cloud
  with negative charges randomly distributed

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Diffuse positive cloud
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5. Nuclear Model

• Ernest Rutherford (1910)
• Performed the famous gold foil experiment

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Set-up
Positive particles
Uranium
Gold foil (0.00006 cm thick)
Detection screen
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Hypotheses

• If Thomson was correct and atoms contained only
  diffuse positive charge then
• Most of these heavy positive particles should go
  right through, right?

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Experiment
Positive particles
Uranium
Gold foil
Detection screen
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Whats going on?

• Most are going straight through
• But some are being deflected
• How much can they be deflected?

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Continuation
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HOLY ATOMS!

• What conclusions can be drawn here?
• There is something inside an atom that is heavy
  enough to deflect a particle straight back
• That something must be positively charged
• It must also be very small

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Conclusions?

• Heavy, dense, positively charged, small piece of
  the atom is.
• The nucleus!!
• Lets watch this animated

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Nuclear Theory

• Nucleus of atom contains _at_99.9 of the mass of
  the atom
• Nucleus is positively charged
• Electrons orbit the nucleus randomly and take up
  _at_99.9 of the space
• Electrons are negatively charged

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Side note

• Notice that Rutherford has no mention of the
  neutron
• Neutron wasnt discovered until 1933 by Chadwick

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6. Planetary Model

• Bohr proved that electrons dont orbit randomly
  around the nucleus
• He postulated that they orbit the nucleus like
  planets do around the sun
• They circle in well-defined energy levels
• Jumping to and from energy levels absorbs or
  releases energy

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Atomic Line Spectra
Expected to see a nice progression of color just
like normal BUT..
Prism or diffraction grating
Gas such as Neon or Hydrogen
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Atomic Line Spectra
Instead he got just a few discrete lines of color!
Prism or diffraction grating
Gas such as Neon or Hydrogen
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WHAT GIVES?

• Why are only a few lines of color seen instead of
  the whole spectrum?
• Bohr postulated that electrons exist at specific
  energy levels or quanta.
• Electrons can only jump from specific energy
  level to specific energy level and not anywhere
  in between.

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Planetary Model
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For Example
OK
Energy level 2
NO!
OK
Energy level 1
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For Example
Energy level 2
To go from 1 to 2 energy must be absorbed or put
into the system of a specific amount
Energy level 1
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Or
Energy level 2
When you drop from a higher energy level to a
lower one energy is released in a specific amount
Energy level 1
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For Electrons
3 ? 1
2 ? 1
Nucleus
3 ? 2
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7. Quantum Mechanical Model

• Heisenberg, Planck, Einstein, etc
• 1900s to today
• The world of the atom is so small that just by us
  looking for small pieces changes where they are
• Thus we cant be sure exactly about these energy
  levels so.

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Electron Cloud Model
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