The History of the Atom

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The History of the Atom
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Table of Contents

• Black Boxes
• Democritus
• Daltons Theory
• Thomsons Electron
• Rutherfords Nucleus
• Goldsteins Protons and Chadwicks Neutron
• Review all three models
• Atomic Symbols

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Black Boxes
Black Boxes are anything that you cannot see
inside
Like a cell phone Or a locked box Or an atom
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The Beginning of the Atom
Find out about it here
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The ancient Greeks (back around 500BC) believed
there were only four elements Earth Air Water Fire
But
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Democritus

• He also lived in ancient Greece but he believed
  differently
• He said that all matter was made up of particles
  that could not be divided any further
• He called these particles
• Atoms

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The Atomic Theory
Learn about it here
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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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Daltons Atomic Theory (1803)

• All matter is made up of tiny indivisible and
  indestructible particles called atoms
• All atoms of one element are exactly alike, but
  they are different from atoms of other elements
• Atoms of different elements can physically mix
  together or can chemically combine into compounds
• Chemical reactions occur when atoms are
  separated, joined, or rearranged.

Click here to see his model
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Daltons Model of the Atom
He believed the atom was a solid sphere
An analogy to his atom would be
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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?
• Was it a light?
• Was it a particle?
• To test this, he brought a magnet close to the
  cathode ray tube to see what would happen.
• Click here to see what happened

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Go on
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• So, the magnet caused the cathode ray to move.
  What does that tell us?
• Would a magnet affect a light from a flashlight?
• Not at all!
• Therefore, the cathode ray must be a particle!

What else did he find
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Thomson also noticed
That the cathode ray was coming out of the
negative end (cathode) of the tube and going
toward the positive end (anode) Therefore,
because opposites attract, he concluded that the
cathode ray must be negative Click here to find
out what he called these
particles
Negative end
Positive end
Negative terminal
Positive terminal
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ELECTRONS!!

• This discovery would alter Daltons model of the
  atom because now we know there is something
  inside it
• But, in addition to these negatively charged
  electrons, there must be something giving it a
  positive charge because the overall charge of the
  atom is neutral (not negative)
• Lets look at Thomsons model of the atom

Actually, George Stoney came up with that name
in 1874 but he never proved their existence
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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
Negative electrons
Why the plum pudding model? What is plum
pudding? Click here to find out
Positive Charge
Positive charge spread across sphere
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Plum Pudding is an English dish sort of like
bread pudding with raisins in it.
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.

(1871-1937)
In it, he shot alpha particles (very small,
very dense, very fast particles) at a thin layer
of gold foil.
Go on
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• He expected all of the alpha particles to go
  straight through because
• It would be like you were shooting bullets at a
  cakeall of the bullets (or alpha particles)
  would easily go straight through the cake (or
  gold foil atoms)

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Alpha particles going straight through
Go on
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But, what he found was this
Alpha particles were scattered by the gold foil
Click here to go back to previous slide if you
want to see what he expected again
Go on
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Or
Go on
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Did you notice how most of the alpha particles
went straight through but a very tiny amount were
deflected at odd angles? That could only happen
if there was something very tiny in the atom that
was dense enough to deflect the alpha particles.
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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Through the Gold Foil Experiment

• Rutherford proved that an atom was
• mostly empty space (because most particles went
  straight through)
• with a very small (only a few particles were
  deflected), very dense (it was hard enough to
  deflect the alpha particles), positively charged
  nucleus in it.

why would it have to be positive?
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• Do you remember what opposites do?
• They attract, right?
• Since the alpha particles are positive, what
  would a negatively charged nucleus cause?
• The alpha particles would be attracted and be
  stuck, rather than deflected out.
• But, a positive nucleus would repel (deflect),
  which is what happened

Lets look at Rutherfords model of the atom
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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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A Rutherford Atom Analogy
In which the peach pit is the nucleus and the
rest of the peach represents the electrons
buzzing around
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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 are given credit for
  finding the neutron

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To Review
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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
Any atom that has gained or lost an electron (or
more than one) will have a charge they are
called ions
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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How about this one
0
41
Ca
20

• Find the
• Atomic number
• Mass number
• Charge
• of p
• of n0
• of e-

Click here for answers
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The answers
0
41
Ca
20

• Find the
• Atomic number
• Mass number
• Charge
• of p
• of n0
• of e-

20 41 0 20 21 20
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