Early History of the Atom
• Anything with
mass and
volume
• Composed of
atoms
Hypothesis
Theory
• An educated guess based
on an observation
• A broad set of observations
that have been supported
with repeated testing
1
(1766 -1844)
• Developed the first
theory of the
structure of the
atom
• “Billiard Ball” Model
of the atom
Dalton’s “Billiard
Ball” Model
All elements are made up of tiny indivisible
particles.
Atoms of the same element are identical. The
atoms of one element are different from the atoms
of another element.
Atoms of different elements chemically combine to
form chemical compounds.
During chemical reactions, atoms are rearranged.
Atoms of one element cannot be changed into the
atoms of another element in a chemical change.
(1856 –
1940)
• Experimented with
cathode ray tubes
• Discovered
negatively charged
electrons
• “Plum Pudding”
model of the atom
Thomson’s “Plum
Pudding” Model
2
Cathode Ray Tube
Negative
Plate
Cathode
Positive
Plate
Deflected Cathode
Ray Tube
(1871 –
1937)
• Experimented with
alpha particles and
gold foil
• Discovered the
positively charged
nucleus (protons)
• “Nuclear” model of
the atom
Rutherford’s
“Nuclear” Model
Rutherford’s Gold Foil Experiment
Close-up Of
Gold Foil Atoms
Alpha Particles Hit
Gold Atom Nucleus And
Are Deflected
3
(1891 –
1974)
• Experimented with
radioactivity
• Discovered the
neutron
4