Transcript History of Atomic Structure

History of Atomic
Structure
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”.
• Why: Aristotle and Democritus used
observation and inferrence to explain the
existence of everything.
Democritus
Aristotle
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
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.
• Why: Building on the ideas of Democritus in
ancient Greece.
John Dalton
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.
J. J. Thompson
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.
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
Quantum Model
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Who: Louis de Broglie
When: 1924
Where: France
What: De Broglie showed that electron beams
could be diffracted or bent as they passed through
a slit much like light could. So, the waves produced
by an electron confined in its orbit about the
nucleus sets up a standing wave of specific
wavelength, energy and frequency, much like a
guitar string sets up a standing wave when plucked.
• Why: He was trying to show why electrons stay in
certain areas around the nucleus of an atom.
Atomic Structure Timeline
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