Transcript History of Atomic Structure

History of Atomic
Structure
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Ancient Philosophy
Who: Aristotle, Democritus
When: More than 2000 years ago (400 B.C.)
Where: Greece
What: Aristotle believed in 4 elements: Earth,
Air, Fire, and Water. Democritus believed
that matter was made of small particles he
named “atomos”. Two prevailing theories.
• Why: Aristotle and Democritus used
observation and inference 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.
Particle Theory
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Who: John Dalton
When: 1803
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.
Particle Theory
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4 Parts of Dalton’s Theory
All elements are composed of indivisible
particles.
Atoms of the same element are exactly alike.
Atoms of different elements are different.
Compounds are formed by joining atoms of
two or more elements.
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. Therefore, the atom must be
composed mostly of positively charged
material. (Plum Pudding Model).
J. J. Thompson
Atomic Structure I
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Who: Ernest Rutherford
When: 1908
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 (nucleus).
• Why: He knew that atoms had positive and
negative particles, so the nucleus must be
positively charged.
Ernest Rutherford
Atomic Structure II
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Who: Niels Bohr
When: 1913
Where: England
What: Proposed that electrons traveled in
fixed orbits 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
Atomic Structure III
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Who: James Chadwick
When: 1935
Where: England
What: .Discovered the neutral particle in the
center of the atom (neutron).
• Why: Chadwick was trying to figure out the
discrepancy of atomic mass not being equal
to the number of protons plus electrons. He
theorized there must be a massive particle
that had no charge and was therefore hard to
find.
Electron Cloud Model
• Electrons travel around the nucleus in
random orbits (no definite paths).
• Scientists cannot predict where they will be
at any given moment.
• Electrons travel so fast, they appear to form
a “cloud” around the nucleus.
• The small, positively charged nucleus is
surrounded by a large space called the
electron cloud.
The Model of the Atom Through Time