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Introduction to the Atomic
Model
Ms. Grobsky
Exploring the Black Box
Imagine you’ve received a present
that you are not allowed to open.
You REALLY want to know what’s
inside!
What might you do to figure
out the contents?
Exploring the Black Box
In your group, try to figure out the contents and
internal structure your present WITHOUT opening it
Note: Do NOT shake the box too violently as the
contents may fall out!
As a group, be sure to answer the questions on the
hand-out!
So What is a Black Box?
A black box refers to any system that has
inner workings that cannot be directly
observed because it is not possible or
feasible
Can you think of any other black box
examples?
An atom is an example of a black box!
The Atomic Model and the Nature of
Science
When scientists wanted to determine the
structure of an atom, they were not able
to actually see inside of the atom
They had to rely on empirical
evidence, indirect evidence,
observation, and inference when
developing their ideas-just as you did
when determining the contents of
your present!
The Atomic Model and the Nature of
Science
Scientists developed a model to express their new
ideas
The understanding of the structure of the atom
has changed many times over the course of
centuries
As new evidence emerged, the ideas of the atomic
model changed
Lecturing about the scientists of the atomic model
isn’t quite as fun and interesting as understanding:
HOW was the atomic model developed?
WHY has the atomic model changed over the years?
The Evolution of the Atomic
Model
Democritus to Rutherford
Mind Catalyst
When asked the question, “What is matter?”, Democritus
responded with the following:
By convention there is color,
By convention sweetness,
By convention bitterness,
But in reality there are atoms and space
•
What do you think he meant by this quote?
•
Nothing exists except atoms and empty space; everything else
is opinion
Democritus
The Greek philosopher
Democritus began the
search for a description of
matter more than 2400
years ago
He asked: Could matter
be divided into smaller
and smaller pieces
forever, or was there a
limit to the number of
times a piece of matter
could be divided?
400 BC
Atomos
His theory: Matter could not
be divided into smaller and
smaller pieces forever,
eventually the smallest
possible piece would be
obtained
This piece would be
indivisible
He named the smallest piece
of matter “atomos,” meaning
“not to be cut”
Atomos

To Democritus, atoms
were small, hard particles
that were all made of the
same material but were
different shapes and sizes

Atoms were infinite in
number, always moving
and capable of joining
together
Democritus vs. The Philosophers
The eminent philosophers
of the time, Aristotle and
Plato, had a more
respected, (and ultimately
wrong) theory
Aristotle and Plato favored
the earth, fire, air and water
approach to the nature of
matter
Their ideas held sway
because of their eminence
as philosophers
The atomos idea was
buried for approximately
2000 years
Then Along Came Dalton’s Model
In the early 1800s, the English
chemist John Dalton
performed a number of
experiments that eventually
led to the acceptance of the
idea of atoms
Dalton’s atomic theory is
comprised of 4 postulates
which were based on chemical
observations made in the lab
This theory is commonly
referred to as the billiard ball
model
Dalton’s Postulate #1
Each element is composed of
extremely small particles called
atoms
Dalton’s Postulate #2
All atoms of a given element are
identical
H
H
Dalton’s Postulate #2
But, the atoms of one element are
different from the atoms of all
other elements
O
H
Dalton’s Postulate #3
Atoms of one element cannot be changed into
atoms of a different element by chemical
reactions
In other words, atoms are neither created nor
destroyed in chemical reactions
They are indestructible!
Dalton’s Postulate #4
Compounds are formed when atoms of more
than one element combine
A given compound always has the same relative
number and kinds of atoms
Dalton’s Atomic Model
This theory became one of
the foundations of modern
chemistry even though
Dalton did not have any
direct evidence for the
existence of atoms!
BUT, it needed work
Which of Dalton’s
principles do you know
to not hold true?
Thomson’s Atomic Model
In 1897, the work of
an English scientist
J. J. Thomson
provided the first
hint that atoms
were made up of
even smaller
particles
Thomson’s Experiment
Thomson studied electrical discharge
through a glass tube pumped almost
empty of neutral air
When a high voltage was applied to the
electrodes, negatively-charged radiation,
called cathode rays, would travel from the
negative electrode to the positive electrode
The path of the cathode rays could be
altered by the presence of a magnetic field
Cathode Ray Tube
Thomson Model
Thomson observed that
cathode rays behaved
identically regardless of
the identity of the
metal used as cathode
material
He described cathode
rays as streams of
negatively-charged
particles which came
from within the atom
• As a result of his work, a
particle smaller than an
atom had to exist
• The atom was divisible!
• Thomson called the
negatively charged
“corpuscles,” today known
as electrons
And...
•
Since the gas was known to be neutral, having no
charge, he reasoned that there must be positively
charged particles in the atom
• Thomson thought the positive (+) and negative (-)
particles were
spread out
in atoms like:
ppuldudmisn
(plums in puddin’)
Charge and Mass of the Electron
Thomson also determined the charge to mass
ratio of an electron to be 1.76  108 C/g
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!
Millikan’s Oil Drop Experiment
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
Millikan’s Oil Drop Apparatus
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!
And Then There was
Rutherford…
The work of Thomson and Millikan provided growing
evidence that the atom is composed of smaller particles
In 1908, the English physicist Ernest Rutherford was
hard at work on an experiment that seemed to have
little to do with unraveling the mysteries of the atomic
structure
Time for Rutherford’s
Discovery Activity!