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Atoms and Their
Structure
Early Greek Theories
Democritus
• 400 B.C. - Democritus crushed substances in
his mortar and pestle
• There eventually came a point where he could
• not crush the substance anymore (still kept its
integrity)
• This became know as the atom. (could not be
divided)
Name atomos - indivisible
Democritus’ Model
solid indestructible sphere
John Dalton
 1800
-Dalton proposed a modern atomic model
based on experimentation not on pure reason.
•
•
•
•
All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine
in constant ratios to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the law of conservation of
mass (atoms are neither created nor destroyed)
and the law of constant composition (elements
combine in fixed ratios).

+
Carbon, C
Mass 12.0
Oxygen, O
Mass 16.0
Carbon Monoxide, CO
Mass 12.0 + Mass 16.0
Law of Constant Composition
+
Carbon
1
=
Carbon Monoxide,
1:1
Oxygen
1
+
=
Carbon
Oxygen
1
2
Carbon Dioxide,
1:2
John Dalton’s Model
solid indestructible sphere
Parts of Atoms
 J.
J. Thomson - English
physicist. 1897
 Used a piece of equipment
called a cathode ray tube.
Thomson’s Experiment
Voltage Source
-
+
Metal Disks
Thomson’s Experiment
Voltage Source
-
+
 Passing
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage Source
+
 By
adding an electric field he
found that the moving pieces were
negative
Thomson’s Model
Found the
electron.
 Said the atom
was like plum
pudding.
 The atom is a
sphere with a
positive charge,
where negatively
charged pieces
are embedded

Rutherford’s Experiment
Ernest Rutherford English
physicist. (1910)
 Used Alpha particles - (He atom,
positively charged)
 Shot them at gold foil which can be
made a few atoms thick.

Lead
block
Uranium
Flourescent
Screen
Gold Foil
He Expected
 The
alpha
particles to
pass through.
 Because…
 The positive
charges were
spread out
evenly. (atom
is neutral)
He Expected
What he got……
What he got…..
+
Rutherford’s Model
 Atom
is mostly
empty.
 Small dense,
positive piece
at center.
Bohr Model
 Electrons
move in definite orbits
around the nucleus, (like planets
circle the sun)
 These orbits, or energy levels, are
located at certain distances from
the nucleus.
 Energy separates one level from
another.
Bohr’s Model
Nucleus
Electron
Orbit
Bohr’s Model
Modern View / Wave
Model
 Nucleus-
protons and
neutrons.
 Electron cloudregion where
you ‘might’ find
an electron.
The Wave Model
 It
is impossible to determine the
exact location of an electron. The
probable location of an electron
is based on how much energy
the electron has.
The Wave Model