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History of Atomic Structure
Ancient Philosophy
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
“atomos” (greek word meaning “unable to
• Why: Aristotle and Democritus used observation
and inference to explain the existence of
Particle Theory
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
• Atoms join to form compounds
• Why: Building on the ideas of Democritus in ancient
John Dalton
Discovery of Electrons
Who: J. J. Thompson
When: 1897
Where: England
What: Thompson discovered that electrons were
smaller particles of an atom and were negatively
• Why: Came up with plum pudding model which
said the atom was a solid + charged sphere w/
negatively charged electrons embedded in it.
J. J. Thompson
J J Thomson (Plum pudding model)
Atomic Structure I
Who: Ernest Rutherford
When: 1911
Where: England
What: Conducted the gold foil experiment and
discovered the nucleus.
• Why: Decided that the atoms were mostly empty
space, but had a dense + charged central core
(nucleus) surrounded by negatively (-) charged
Ernest Rutherford
Atomic Structure II
Who: Niels Bohr
When: 1913
Where: England
What: Proposed that electrons traveled in fixed
paths (solar system model) around the nucleus.
Scientists still use the Bohr model to show the
number of electrons in each orbit around the
• Why: Bohr was trying to show why the negative
electrons were not sucked into the nucleus of the
Niels Bohr
Atomic Structure III
• Who: Erwin Schrodinger
• When: 1926
• What: Proposed the Electron Cloud Model which
said electrons travel around the nucleus in
random orbits.
• Why: 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