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The Evolution of
the Atomic
Model
Democritus to Rutherford
The Atomic Theory of Matter
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Philosophers from the earliest times, like
Democritus, described the material world as
made up of tiny, indivisible particles called
atoms
Plato and Aristotle formulated the notion that
there can be no ultimately “atomic” view of
matter
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This dominated Western ideas for many
centuries
The notion of an atomic theory of matter
reemerged during the 17th century as a result
of the work of John Dalton
John Dalton and the Atomic
Theory of Matter

Dalton’s atomic theory is comprised of 4 postulates
which were based on chemical observations made
in the lab:
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Each element is composed of extremely small particles
called atoms
All atoms of a given element are identical, but the
atoms of one element are different from the atoms of all
other elements
Atoms of one element cannot be changed into atoms
of a different element by chemical reactions; atoms are
neither created nor destroyed in chemical reactions
Compounds are formed when atoms of more than one
element combine; a given compound always has the
same relative number and kinds of atoms
The Atomic Theory of Matter

Dalton’s theory explains several laws of chemical
combination that were known during his time:

Law of Constant Composition
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Law of Conservation of Mass
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In a given compound, the relative numbs and kinds of
atoms are constant
Based on
In chemical reactions, the total mass of materials present
before and after is the same
Dalton used his theory to deduce the Law of Multiple
Proportions which states:

If two elements A and B combine to form more than one
compound, the mass of B that can combine with a
given mass of A are in a ratio of small whole numbers
The Discovery of Atomic
Structure
 The
ancient Greeks were the first to
postulate that matter was indivisible
 Even Dalton did not have any direct
evidence for the existence of atoms!
 Later, scientists realized that the atom
consisted of subatomic, charged particles
 What landmark discoveries led to this
conclusion?
Cathode rays and electrons

A cathode ray tube (CRT) is a hollow vessel with
an electrode at either end

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A high voltage is applied across the electrodes
The voltage causes negative particles to move
from the negative electrode to the positive
electrode
The path of the electrons can be altered by the
presence of a magnetic field
J.J. Thomson described cathode rays as streams of
negatively charged particles, called corpsicles
which later become known as electrons!

He also determined the charge to mass ratio of an
electron to be 1.76  108 C/g
Cathode Ray Tube
Charge and Mass of the
Electron
 Once
the charge-to-mass ratio of the
electron and charge of the electron was
known, the mass of the electron could be
deduced!
 In 1909, Robert Millikan succeeded in
doing just that with his oil-drop
experiment!
Charge of Mass of the
Electron

Consider the following
experiment:
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Oil drops are sprayed above
a positively charged plate
containing a small hole
As the oil drops fall through
the hole, they are given a
negative charge
Gravity forces the drops
downward
The applied electric field
forces the drops upward
When a drop is perfectly
balanced, the weight of the
drop is equal to the
electrostatic force of
attraction between the drop
and the positive plate
Charge and
Mass of Electrons

Using this experiment, Millikan determined the
charge on the electron to be 1.60  10-19 C

Knowing the charge to mass ratio, 1.76  108
C/g, Millikan calculated the mass of the
electron as 9.10  10-28 g

With more accurate numbers, we get the
mass of the electron to be 9.10939  10-28 g!
The Discovery of radioactivity and its
Effect on Atomic Structure
 Consider the following experiment:
• A radioactive substance is placed in a shield
containing a small hole so that a beam of
radiation is emitted from the hole
• The radiation is passed between two electrically
charged plates and detected
• Three spots are noted on the detector:
• A spot in the direction of the positive plate,
• A spot which is not affected by the electric field
• A spot in the direction of the negative plate
The Discovery of radioactivity and its
Effect on Atomic Structure
The Discovery of radioactivity and its
Effect on Atomic Structure
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What did these observations mean?

A high deflection towards the positive plate corresponds
to radiation which is negatively charged and of low
mass
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No deflection corresponds to neutral radiation
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This is called β-radiation which consists of electrons
This is called γ-radiation which consists of energy
Small deflection towards the negatively charged plate
corresponds to high mass, positively charged radiation

This is called α-radiation which consists of positivelycharged helium nuclei
The Discovery of radioactivity and its
Effect on Atomic Structure
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
From the separation of
radiation, we conclude
that the atom consists of
neutral, positively, and
negatively charged
entities
Thomson assumed all
these charged species
were found in a sphere –
similar to the image at
right

Like a chocolate chip
cookie or plum pudding
Rutherford and the Nuclear Atom:
The Gold-Foil Experiment
 Ernest
Rutherford carried out the following
experiment:
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A source of a-particles was placed at the
mouth of a circular detector
The a -particles were shot through a piece
of gold foil
Most of the a-particles went straight
through the foil without deflection
Some a-particles were deflected at high
angles
Rutherford and the Nuclear Atom
Rutherford and the Nuclear
Atom


If the Thomson model of the atom was correct,
then Rutherford’s result was impossible!
So, what do these observations mean?

In order to get the majority of a-particles through a
piece of foil to be undeflected, the majority of the
atom must consist of a low mass, diffuse negative
charge - the electron!

To account for the small number of high
deflections of the a-particles, the center or nucleus
of the atom must consist of a dense positive
charge!
The Nuclear Atom

Rutherford modified
Thomson’s model as
follows:

Assume the atom is
spherical but the
positive charge must
be located at the
center, with a diffuse
negative charge
surrounding it