Atomic Structure History

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Atomic Structure History

• Chemistry 338

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1. Pre-Atomists c. 350 BC

• Aristotle- believed everything was composed of
  earth, air, fire, water and added the 5th element
  ether

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Democritus c. 400 BC

• 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. In later years

• 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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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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-
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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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(No Transcript)
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Discover of Radioactivity

• Chemistry 332

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Where are we?

• This story starts in 1895 with the work of the
  German physicist Wilhelm Roentgen.
• He was working with a Cathode Ray tube and
  noticed something very interesting...

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Cathode Ray Tube?
(With a fluorescent screen the energy path glows
green)
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So What?
He was working on his experiment on this side of
the room and had a different experiment across
the room
Chem Rules!
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He then flipped the switch...
Chem Rules!
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So he turned it off...
And the Cathode ray tube went dead
Chem Rules!
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So he tried it again!
Chem Rules!
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Conclusion?

• The Cathode Ray tube was giving off some sort of
  energy in all directions!!!!
• Roentgen next thought, I wonder how powerful
  these energy rays are?
• So he took the fluorescent screen into the next
  room, came back and flipped the switch.

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The Set-Up
Fluorescent screen
Cathode ray tube
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Conclusion
Fluorescent screen
The tube must be giving off POWERFUL invisible
energy
Cathode ray tube
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What are these?

• The rays of energy were invisible
• They were strong enough to penetrate the wall
• They were unprecendented
• So he called them.
• X-Rays

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So Roentgen Discovered X-Rays

• X-Ray machines began popping up everywhere
• Even shoe stores had them so you could see your
  feet bones
• Some thought they were the Devils tool
• More importantly they led to further studies

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Science is moving faster now...

• Its now 1896, one year after Roentgens work
• Antoine Henri Becquerel has heard of the X-Ray
  and its link to fluroescence and decides to do
  his own experiments with phosphorescence

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His Hypothesis

• Some phosphorescent materials emit X-rays when
  exposed to high energy
• These x-rays can penetrate substances but are
  ultimately just like visible light (just stronger)

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His Set-Up
Sunlight
Photographic film
Would the film be exposed?
Yes
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But.
Sunlight
Photographic film
Would the film be exposed?
No
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But, but.
Sunlight
Photographic film wrapped in thick black paper
What happens to the phosphorescent material in
the sunlight?
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This glowing gives off energy

• If the energy was just visible light, would the
  photographic film be exposed?
• No, its wrapped in black paper the normal light
  cant penetrate
• If the energy also contained X-Rays, would the
  photographic film be exposed?
• Yes, X-rays are just like light but can easily
  penetrate the papter.

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What was the result?

• Sure enough, the photographic paper was exposed!
• This meant that phosphorescent materials, when
  glowing due to the energetic effects of the sun,
  give off X-rays!

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So what?

• Really nothing new happened there with that
  experiment
• But on Feb 26th, Becquerel decided to repeat the
  experiment but couldnt due to cloudy skies over
  Paris.

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Drats!

• Becquerel took the phosphorescent materials and
  the photofilm wrapped in black paper and stuck
  them in a drawer.
• They stayed there, undisturbed in the dark until
  March 1st four full days

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Lets Review

• Becquerels conclusion was that sunlight made the
  phosphorescent material glow
• This gave off penetrating x-rays
• This caused the photofilm to be exposed even
  though it was wrapped in black paper
• Should the photofilm, after sitting the dark,
  wrapped up in black paper be exposed?
• No, theres no light to cause the phosphorescent
  material to glow and thus give off x-rays

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Just out of curiosity

• Becquerel developed the photographic film anyway.
• What did he find?

• The photofilm was exposed just as much as his
  previous experiment!
• What does this mean?

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Sunlight doesnt matter

• The phosphorescent material was spontaneously
  giving off energy regardless of the sun
• This energy was invisible
• It was also strong enough to penetrate the black
  paper
• Were these X-rays or some other energy?

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How many substances do this?

• Marie Curie (a student of Becquerel by the way)
  and Pierre Curie spent years isolating substances
  that exhibit these properties
• But what are they?

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One last experiment

• Ernest Rutherford, using Becquerels ideas and
  material isolated by the Curies performed an
  experiment on the X-rays found by Roentgen as
  well as the energy found by Becquerel

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His Set-Up

Strong magnet
-
X-ray or other energy source
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The results!

Strong magnet
-
What charge are X-rays, then?
X-rays
Neutral, just like light
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The results!

Strong magnet
-
What charges are these?
The energy discovered by Becquerel and the Curies
Positive, negative, and neutral
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What is this?

• This mysterious energy given off by the
  fluorescent substances was of three different
  types
• It was given off spontaneously
• It was invisible
• It was of very high energy
• What is it?

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Ill give you a hint

• The fluorescent ore used by Becquerel was
• K2UO2(SO4)2
• Notice anything unusual about that formula?
• Check out the center atom.
• What do we know about that element?

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Radioactivity!

• So what did this have to do with X-rays or
  fluorescent materials?
• Nothing!
• Except that without those experiments,
  radioactivity would not have been discovered so
  early.