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EARLY MODELS OF THE ATOM
Models of Matter
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a model is a tentative description of a
system or theory that accounts for all of
its known properties
models are invented (usually) to
interpret the invisible and relate
experimental results
1. The Four Elements
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An Ancient Greek model (5 BCE), based
on logic
Matter is composed of four elements:
air, fire, earth and water
An atom is a single indivisible particle
2. John Dalton’s Atomic
Theory (1803)
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based on experimentation,
Experimental data showed:
- elements combine in the same
percent by mass each time the same
compound is formed: atoms of
different elements combine in fixed
proportions
Dalton’s Theory:
1. all matter is made of small spherical particles
called atoms
2. each element is made of its own type of
atom
3. atoms of different elements have different
properties
4. atoms of different substances can combine in
constant ratios to form new substances
Law of Constant Composition:
 atoms of different elements can be
chemically combined in a fixed whole
number ratio to form compounds, ex.
H2O
5. atoms cannot be created or destroyed
during chemical or physical changes
Law of Conservation of Mass:
- the mass of the reactants in a
chemical reaction is equal to the mass
of the products
6. Atoms can neither be subdivided nor
changed to one another
atom
-the smallest particle of an element that
has the properties of that element
BUT
 studies on static charging of objects
were not falling into line with this model
 static charge was shown to be the
result of charged particles
3. Thomson’s Atomic Theory
(1897)
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used a cathode ray tube to energize
electrons
experimental data showed:
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charged particles originated from the gas
inside the tube
charged particles are deflected away from
a negatively charged object, but towards a
positively charged object.
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
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+
Passing an electric current makes a beam
appear to move from the negative to the
positive end.
Thomson’s Experiment
Voltage source
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By adding an electric field
Thomson’s Experiment
Voltage source
+
 By adding an electric field, he found that
the moving pieces were negative
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No matter what the gas used in the
cathode-ray tube, or the metal used for
the electrodes, the beam of particles
reacts the same way. (indicates that the
particles are the same for all atoms)
Atoms are usually neutral, or not
charged
Thomson’s Model
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Found the electron.
Couldn’t find
positive (for a
while).
Said the atom was
like plum pudding.
A bunch of positive
stuff, with the
electrons able to be
removed.
Theory: “Plum Pudding Model”
 Atoms are composed of smaller particles.
 These particles are the same for all different
types of atoms.
 These particles are negatively charged and
are called electrons.
 Electrons are embedded throughout the
uniform sphere of positive charge to make up
a neutral atom.
 Matter is naturally neutral except when it
gains or loses electrons
4. Rutherford’s Gold Foil
Experiment (1911)
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Rutherford (at McGill) used very small
positive particles (alpha particles:
positive helium nuclei) and a
fluorescent screen detector to see the
particles
Since the mass is evenly distributed in
gold atoms alpha particles should go
straight through.
experimental data showed:
 Alpha particles (+ve) were shot at a thin
sheet of gold
 Most alpha particles passed through
unaffected
 Some alpha particles were deflected at large
angles. Some are even reflected back.

- Thomson's model did not explain the
deflection, so …
Lead
block
Uranium
Fluorescent
Screen
Gold Foil
What he expected
What he got
Because
Because, he thought the mass was
evenly distributed in the atom.
How he explained it
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Atom is mostly empty space
Small dense,
positive region
at center.
Alpha particles
are deflected by this
region if they get close
enough.
+
+
Theory
1. an atom is made of mostly empty
space
2. all the positive charge and most of the
mass are in a tiny center of the atom:
the nucleus
3. electrons surround but do not fill the
rest of the atom
Modern View
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The atom is
mostly empty
space.
Two regions
Nucleus- protons
and neutrons.
Electron cloudregion where you
might find an
electron.
Symbolic representation of
elements
Name
Symbol
Charge
Mass
Location
proton
p
+1
1 a.m.u.
nucleus
neutron
n
0
1 a.m.u.
nucleus
electron
e-
-1
1/1837
a.m.u.
energy levels
around nucleus
Sub-atomic Particles
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Z - atomic number = number of protons
determines type of atom.
A - mass number = number of protons
+ neutrons.
Number of protons = number of
electrons if neutral.
Symbols
A
X
Z
23
Na
11
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Elements are represented by the
symbols found on the periodic table
The atomic number of the element is
written as a subscript before the
symbol, it represents the number of
protons (which is equal to the number
of electrons in a neutral atom)
The atomic mass number is written as a superscript
before the symbol
ie Carbon
atomic no. = 6
atomic mass no. = 12
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The atomic mass of the element represents the
umber of protons plus the number of neutrons in the
atom
The number of neutrons in an element can be
determined by subtracting atomic no. from atomic
mass no. (Carbon has 12 - 6 = 6 neutron.)