Transcript File - Mc Guckin Science

History of Atomic
Structure
Democritus
Aristotle
Ancient Philosophy
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Who: Aristotle, Democritus
When: More than 2000 years ago
Where: Greece
What: Aristotle believed in 4 elements: Earth,
Air, Fire, and Water. Democritus believed
that matter was made of small particles he
named “atoms”.
Alchemists
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Who: European Scientists
When: 800 – 900 years ago
Where: Europe
What: Their work developed into what is now
modern chemistry.
• Why: Trying to change ordinary materials
into gold.
Alchemic Symbols
John Dalton
Particle Theory
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Who: John Dalton
When: 1808
Where: England
What: Described atoms as tiny particles that
could not be divided. Thought each element
was made of its own kind of atom.
J. J. Thompson
Discovery of Electrons
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Who: J. J. Thompson
When: 1897
Where: England
What: Thompson discovered that electrons
were smaller particles of an atom and were
negatively charged.
• Why: Thompson knew atoms were neutrally
charged, but couldn’t find the positive
particle.
Ernest Rutherford
Atomic Structure I
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Who: Ernest Rutherford
When: 1911
Where: England
What: Conducted an experiment to isolate
the positive particles in an atom. Decided
that the atoms were mostly empty space, but
had a dense central core.
• Why: He knew that atoms had positive and
negative particles, but could not decide how
they were arranged.
He fired Alpha particles at an extremely
thin gold foil.
• He expected them to go straight
through with perhaps a minor
deflection.
• Most did go straight through, but to
his surprise some particles bounced
directly off the gold sheet!
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• What did this mean?
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• Rutherford hypothesized that the
positive alpha particles had hit a
concentrated mass of positive
particles, which he termed the
nucleus.
Rutherford’s Lab in Cambridge
Atomic Structure II
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Who: Niels Bohr
When: 1913
Where: England
What: Proposed that electrons traveled in
fixed paths around the nucleus. Scientists
still use the Bohr model to show the number
of electrons in each orbit around the nucleus.
• Why: Bohr was trying to show why the
negative electrons were not sucked into the
nucleus of the atom.
Electron Cloud Model
• Electrons travel around the nucleus in
random orbits.
• Scientists cannot predict where they will be
at any given moment.
• Electrons travel so fast, they appear to form
a “cloud” around the nucleus.
Electron Cloud Model
Neils Bohr
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Rays or Particles?
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Curie-Joliots in Paris, had bombarded
beryllium with alpha-particles, and found
that very penetrating gamma rays were
emitted. Cockcroft and Walton thought that
similar rays should be observed when they
bombarded light elements with protons.
Unfortunately, the Curie-Joliots were
mistaken in identifying the 'radiation'. In
January 1932 James Chadwick repeated
their experiments and found that the
reaction produced neutral particles,
neutrons, that Rutherford had predicted in
1920. A few months after Chadwick's
discovery Rutherford came to Cockcroft
and Walton and told them they 'ought to
put in a fluorescent screen and get on with
the job'. Rutherford was clearly hoping to
see alpha-particles, and a fluorescent
screen would be the best way to detect
them.
On 14 April 1932 Walton set up the tube
and bombarded lithium with high energy
protons. He then crawled into the little
observation cabin set up under the
apparatus and immediately saw
scintillations of the fluorescent screen. The
reaction was giving off alpha-particles.
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