Download Democritus John Dalton Joseph John (J.J.) Thomson Dimitri

Scientist
Year
Democritus
~460 BC
John Dalton
1805
Contribution
First to develop idea that matter was made
of particles called “atoms”- from the Greek
word “atomos”, which means “indivisible”.
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Proposed atomic theory; an atom is
indivisible, indestructible, tiny
sphere.
Determined that elements have
unique atomic masses.
Atoms combine to form compounds.
Joseph John
(J.J.)
Thomson
1898
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Discovered electrons.
Proposed an atomic model known
as the “Plum pudding Model”: a
sphere of positive charge
containing electrons.
Dimitri
Mendeleev
1869
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Developed the first periodic table
The periodic table was arranged
according to increasing atomic
weight, and left spaces for elements
that were yet to be discovered.
Later versions refined & expanded
the table.
Ernest
Rutherford
1911
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Discovered positively charged
nucleus through the “Gold Foil”
experiment.
Proposed atomic model with tiny
but massive (highly dense) nucleus
with electrons orbiting around it.
Niels Bohr
1913
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James
Chadwick
1932
Erwin
Schrodinger
1933
Model
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Developed atomic model (“Bohr
Model”) that placed electrons in
specific energy levels (orbits)
around the nucleus.
When energized, electrons can
jump from one orbit to another,
then fall back to their stable (a.k.a.
“ground”) level.
Discovered the neutron as a neutral
particle in the nucleus.
Explained why the mass of an atom
could not be attributed entirely to
the mass of protons.
Developed the “Electron Cloud
Model”, which was built off the Bohr
Model.
Described electrons as waves (using
math) with their probable location
viewed as a cloud.
“Billiard Ball” Model