The History of the Atom

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The History of the Atom
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The Atomic Theory
Learn about it here
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• Atomic Theory
• all matter is made up of atoms
• ?an explanation for why matter behaves the way it
  does

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Democritus

• Around 500BC
• Democritus said all matter was made up of
  particles that could not be divided any further
• Called thematomos
• for unbreakable

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Democritus idea of the atom was largely ignored
until an English schoolteacher did some
experiments over 2000 years later, he was
John Dalton (1766-1844)
Click here to learn about his atomic theory
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C. Daltons Atomic Theory (1803)

• Law of Multiple Proportions
• Atoms join in whole number ratios to form
  compounds

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Daltons Atomic Theory (1803)

• Law of Conservation of Mass
• Atoms are neither created nor destroyed in any
  chemical reactionthey can only be rearranged.

Click here to see his model
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Daltons Model of the Atom
Because he believed that atoms could not be split
apart, he thought the atom was a solid sphere
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D. Thomsons Cathode Ray Tube Experiments and the
Discovery of the Electron
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The Electron

• JJ Thomson (1856-1940) used the CATHODE RAY TUBE
  to show that the atom was made up of electrons in
  1897

Click here to listen to him talk about the
electron
Click here to see a cathode ray tube
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Go on
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• But, what was that green light?
• light?
• a particle?
• To test it, he used a magnet
• Click here to see what happened

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Go on
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• The magnet caused the cathode ray to bend, so
• the cathode ray must be a?

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• particle because light isnt bent by magnets

What else did he find
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Thomson also used a spinning wheel
And the wheel turned (so it was definitely a
particle) If the wheel turned in the direction
indicated, which side is the particle going
to? If it is being attracted to that side, what
is the charge on the particle?
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Thomson also used a spinning wheel
it must be traveling toward the positive
end and since opposites attract, the charge
on the particle must be negative Click here to
find out what he called these particles
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ELECTRONS!!

• This discovery would alter Daltons model of the
  atom because now we know there is something
  inside it negatively charged electrons
• But, the overall charge of the atom is neutral,
  so what else must be in it?

Actually, George Stoney came up with that name
in 1874 but he never proved their existence
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• Positive charges (to cancel out the negative
  charges of the electrons)
• Lets look at Thomsons model of the atom

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Thomsons Plum Pudding Model of the Atom
He believed the atom was made of positively
charged stuff with negatively charged particles
scattered throughout
Why the plum pudding model? What is plum
pudding? Click here to find out
Positive Charge
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Plum Pudding is an English dish sort of like
bread pudding with raisins in it.
But
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An American analogy to his atom would be
is like
Even though we could call it the chocolate chip
cookie model, well still refer to it as the
Plum Pudding model in class
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• Rutherford and the Nucleus

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Ernest Rutherford

• In 1908, Rutherford performed the Gold Foil
  Experiment as he experimented with radiation.

(1871-1937)
In it, he shot alpha particles (very small,
very dense, very fast particles) at a thin layer
of gold foil.
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• He expected
• the particles would go straight through
• Because
• Thomsons model had nothing dense enough in
  it to deflect the particles

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Alpha particles going straight through
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But, what he found was this
Alpha particles were scattered by the gold foil
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want to see what he expected again
Go on
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Or
Go on
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Most of the alpha particles went straight
through A few particles were deflected. Why?
Or, to go on
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Like this
To see it a different way Or, to go on
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To recap the Gold Foil Experiment
Go on
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Rutherford showed that an atom was

• mostly empty space why? (answers on next slide)
• with a nucleus that was
• very small
• very dense
• positively charged

why would it have to be positive?
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Rutherford showed that an atom was

• mostly empty space because most particles went
  straight through
• with a nucleus that was
• very small only a few particles were deflected,
• very dense hard enough to deflect the alpha
  particles,
• positively charged it deflected the positively
  charged alpha particles and we know that like
  charges repel

why would it have to be positive?
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Rutherfords Nuclear Model of the Atom
He believed the atom was made of a tiny
positively charged nucleus with negatively
charged particles orbiting it
Another viewmore like you will have to draw it
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Or
If the atom were Paul Brown Stadium, the nucleus
would be a pea on the fifty-yard line
Click here for an analogy
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Other important discoveries

• In 1886, Goldstein discovered the proton (which
  is located in the nucleus)
• In 1932, Chadwick discovered the neutron (also
  located in the nucleus)
• Irene and Joliet Curie also share credit for
  finding the neutron

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To Review
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IV. Atomic Symbols

• Now, we can determine the number of each of these
  particles if we know the atomic number and mass
  number.
• Atomic Number of protons
• Mass Number of protons of neutrons
• Charge of protons - of electrons

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Reading symbols

• Symbols
• Contain the symbol of the element, the mass
  number, the atomic number and the charge

Mass number
Charge
X
Atomic number
Try some problems
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Try it

• Find the
• Atomic number
• Mass number
• Charge
• of protons
• of neutrons
• of electrons

Click here for answers
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The answers

• Find the
• Atomic number
• Mass number
• Charge
• of protons (p)
• of neutrons (n0)
• of electrons (e-)

9 19 -1 9 10 10
Try another problem
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Reading symbols

• Symbols
• Contain the name of the element, and the mass
  number

Name of element
Mass number
oxygen-15
-it does not tell the atomic number
-assume the charge is neutral
Try some problems
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Other Symbols
oxygen-15

• Find the
• Atomic number
• Mass number
• Charge
• of protons
• of neutrons
• of electrons

Click here for answers
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Other Symbols
oxygen-15

• Find the
• Atomic number 8
• Mass number 15
• Charge 0
• of protons 8
• of neutrons 7
• of electrons 8

Click here for next
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How about this one

• Find the
• Atomic number
• Mass number
• Charge
• of protons
• of neutrons
• of electrons

Click here for answers
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The answers
2
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Ca
20

• Find the
• Atomic number
• Mass number
• Charge
• of protons
• of neutrons
• of electrons

20 41 2 20 21 18
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