Transcript History of Atomic Structure

Bellwork 10/12/06
How long have people been interested in
understanding matter and its structure?
A. Thousands of years
B. Hundreds of years
C. A few years
D. Never
History of Atomic
Structure
Ancient Philosophy
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Who: Democritus
When: 460-370 b.c.e.
Where: Greece
What: Democritus believed that matter was made
of small particles he named “atoms”. Said they
were solid, homogeneous, and couldn’t be divided
• Atoms of different elements have different sizes,
shapes and properties
• Why: Democritus used observation and inference
to explain the existence of everything.
Aristotle
• When: 334-322 B.C.E.
• Aristotle believed in 4 elements: Earth, Air,
Fire, and Water.
• No such thing as empty space
Democritus
Aristotle
Particle Theory
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Who: John Dalton
When: 1808
Where: England
What: Described atoms as tiny particles that could
not be divided or destroyed. Thought each element
was made of its own kind of atom that were exactly
the same in size and mass.
• Atoms of elements can combine to form
compounds
• Why: Building on the ideas of Democritus in ancient
Greece.
John Dalton
Sir William Crookes
• Cathode Ray tube
• Found that electricity
had a negative charge
• Late 1800’s
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. Plum pudding model
• Why: Thompson knew atoms were neutrally
charged, but couldn’t find the positive
particle.
J. J. Thompson
Robert Millikan
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Early 1910s
Found the charge and mass of electrons
9.1x1028
.0000000000000000000000000091g
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. Gold foil
experiment.
• Why: He knew that atoms had positive and
negative particles, but could not decide how
they were arranged.
Ernest Rutherford
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.
Niels Bohr
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
Atomic Structure Timeline
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