Download File - 8th Grade Physical Science

Introduction To Atoms
Section 1: Development of The
Atomic Theory
Atoms…
…Are the smallest particles into which
an element can be divided and will
remain the same substance
…Make up elements (which combine to
form compounds)
…often called the building blocks of
matter
Atoms…
…Were not seen prior to 1981 when the
scanning tunneling microscope was invented
…comes from the Greek word atomos,
meaning indivisible
However…
The atomic theory has been around for more
than 2000 years!
Refresh your memory…
What’s a theory?
A theory is an explanation for a broad
range of hypotheses and observations
that have been supported by testing
Let’s Tour The History of
The Atom
First Stop…Greece
Democritus proposed
(around 440 BC)…
…that if a person continued to cut a coin
in half over and over again, they
would eventually end up with an
uncuttable piece
…the name for the atom
Aristotle disagrees. He
thought (around mid 300
BC, after Democritus)…
…particles could be divided indefinitely
It’s All Greek To Me!
So Who Thought Up The
Atomic Theory?
Let’s move to Britain!
…and about 2000 years
have passed.
…in the 1700’s, scientists have learned
that elements combine in specific
proportions based on mass
What are combined elements with a
constant mass ration called?
(Hint: Ch 4)
Compounds!
Dalton’s Theory Stated…
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All substances are
made of atoms. Atoms
are small particles that
cannot be created,
divided, or destroyed
Atoms of the same
element are exactly
alike, and atoms of
different elements are
different
Atoms join with other
atoms to make new
substances
Meet Dalton
So, is the atom really the
smallest particle ?
We thought so until 1897 (yes, just 110
years ago!) Another British scientist,
J.J.Thomson, discovered that there
were small particles inside the atom,
therefore, the atom could be divided
into smaller parts!
Thompson experimented
with the cathode ray tube
and discovered…
Thompson concluded,
And he is credited with the discovery of
the electron!
He is known for the “plum pudding” or
“chocolate chip cookie” model of the
atom.
A Few Years Later…
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A student of Thompson’s named
Ernest Rutherford decided to check
out Thompson’s theory himself so he
set up an atomic shooting gallery
Meet Rutherford
Bye-Bye Plum Pudding!
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Rutherford came up with his own
model of the atom
He is credited with the discovery of
the proton and nucleus
Rutherford's model came to be known
as the “solar system” model
Jumping Electrons!
Niels Bohr (who
worked with
Rutherford)
suggested that
electrons travel
around the
nucleus in
definite paths
Jumping Electrons!
Bohr also proposed
that the electrons
could jump
between paths
Electron Clouds and
Current Theory

The exact
movement of an
electron cannot
be predicted but,
according to
current theory,
there are regions
of the atom
where the
electrons are
more likely to be
found. These
are called
electron clouds.
Chadwick…1932

Credited with the discovery of the
neutron which is located in the
nucleus with the protons
The Atom Through Time…
History of the periodic
table…
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Mendeleev – created the first periodic
table, ordered by increasing atomic
mass
Mosley – rearranged the periodic table
in order of increasing atomic #, this is
the way the current table is arranged
Let’s summarize
In what two parts of an atom is most of
its mass located?
Most of the mass of an atom is located in its nucleus
Let’s summarize
What are the two differences between the
atomic theory described by Thompson and
described by Rutherford?
The differences in the theories are that Thompson’s
model had the negatively charged particles in the
positive material but Rutherford’s model had them
moving around the positive material. Thompson’s
model does not have a nucleus in the atom but
Rutherford’s does.
Let’s summarize
Identify the difference in how Bohr’s
theory and the modern theory
describe the location of electrons.
Bohr’s theory was that electrons move in definite paths
around the nucleus. The modern theory states that the
path of an electron cannot be known. Only the areas of
the atom where electrons are likely to be found can be
described.
Draw an atom:
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Now that we know a little more about
the atom lets draw one.
– First, close your notebook and try from
memory!
– Draw one atom you remember from
today and tape it to the correct cabinet
around the room