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Atom: The smallest particle into which an element can be divided
and still be the same substance.
A.) Democritus
 440 B.C.
 “Uncuttable” particle – named it the atom
 All atoms are:
 Small
 Hard
 Made from a single material formed into
different sizes & shapes
 Always moving
 Join together to form different materials
-- Aristotle
384 – 322 B.C.
Disagreed w/ Democritus; people believed
B.) Dalton
 Around 1800
 Performed experiments
 Published Atomic Theory in 1803:
 All substances made from atoms, which
cannot be created, divided, or destroyed
 Atoms of same element exactly alike
 Atoms join to make new substances
C.) Thomson
 Around 1900
 Cathode-Ray Tube experiment
 Identified Negative Corpuscles – now called
 Plum Pudding Model – electrons scattered
D.) Protons identified in 1902
E.) Rutherford
 1909
 Gold Foil experiment
 1911 – new model
 Center = Positive nucleus, dense, most of
the mass here
F.) Bohr
 1913
 Electrons orbit in specific levels
 Electrons can jump levels with energy
G.) Electron Cloud Theory
 Current Theory
 Regions where electrons are likely to be found