Download Chemistry Powerpoint #5 ATOMIC STRUCTURE THEORIES

Atomic Structure
Hypothesis –
First Models Of Matter
There were many scientist that
contribute to the development of the
first models of matter
1. Aristotle
4. J.J. Thomson
2. Democritus
5. Rutherford
3. John Dalton
6. Bohr
Dalton’s Atomic Model
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Even today, basic principles of
his theory are still true.
His model was modified to
explain new observations of
later chemists
Developed by John Dalton
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He is known as the father of the Atomic
Theory.
He formulated four main points concerning
atoms:
Memorize
1) All matter is composed of tiny
indivisible particle called atoms.
2) All atoms of the same element are
identical (same).
3) Atoms of different elements are
different.
4) During a chemical reaction atoms
combine to form new products.
J.J. Thomson’s Atomic Model
Thomson discovered the electron while
experimenting with cathode rays.
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Electrons have a
negative charge
Thomson believed:
The atom was a positive sphere with
electrons embedded in it like raisins in a
plum pudding.
Cathode Rays
A cathode ray tube is
a sealed evacuated
glass tube.
Cathode rays are beams of tiny, negatively
charged particles called electrons that
flow through this tube.
J.J Thompson used these tubes to discover
the various properties of cathode rays
(electrons)
Properties Of Cathode Rays
1. The cathode rays travel in
a straight line.
2. The cathode rays drive a small
propeller located in their path.
This tells us that cathode rays have
mass.
Properties Of Cathode Rays
3. The cathode rays cause the
shadow of the object in their
path to be projected on a
fluorescent screen.
Properties Of Cathode Rays
4. The cathode rays are deflected
towards the positively charged
plate.
This tells us that cathode rays are
negative.
RUTHERFORD’S ATOMIC MODEL
Ernest Rutherford also developed
his own atomic theory
HE BELIEVED:
the atom had a central positive nucleus
surrounded by negative electrons floating
around in a cloud like structure
Rutherford’s Atomic Model
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Rutherford used the ideas of
radioactivity to carry out experiments
to help him better define the atomic
model.
His experiment involved:

Exploring matter by shooting alpha
particles against thin sheets of metal.
A FEW particles are sharply
deflected
SEVERAL particles pass through the
gold foil & were slightly deflected
Alpha
Particle beam
Thin sheet of
Gold foil
Radium
in Lead Block
MOST particles passed through the
Fluorescent Screen
gold foil & were not deflected
Fluorescent Screen
MOST went through
Alpha
Particles
empty space
Conclusions
from
Rutherford’s
Experiment:
FEW approach
nucleus & bounced
back
Gold Atom
3 Main Observations
SEVERAL were
deflected by the nucleus
OBSERVATIONS
1.MOST (almost all) went
through the gold foil &
were not deflected
2.
FEW (not many)
bounced directly back
3.
SEVERAL (some)
passed through the gold
foil & were deflected
CONCLUSIONS
Atom contains a significant
amount of empty space
The atom’s dense core
(nucleus) must be very small
The nucleus must be positive
(law of repulsion)
Rutherford Model
Rutherford proposed
a new
model based
on his findings
and
Thomson's Model.
Rutherford’s Model
placed electrons orbiting,
like a cloud,
around a dense,
positively
charged nucleus.
Nucleus
Electrons
Which diagram is Thompson’s and which
diagram is Rutherford’s?
_
_ _
_
+
+
_
_
_
_
+ + _
+_ _
+
+ _
Rutherford believed the nucleus was
POSITIVE or NEGATIVE?
Bohr – Rutherford Atomic
Model:
NEIL BOHR

proposed modification to
Rutherford Model

proposed that electrons can only
circle the nucleus at very specific
distances (called orbits)
Bohr & Rutherford combined
theirtheories to create the current
simplified atomic model

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Thomson’s Model is known as the
plum pudding model. He also
discovered the electron.
Rutherford’s Model


Positive nucleus surrounded by a fuzzy
cloud
Electrons not organized

Bohr – Rutherford Model
states that here is a nucleus that is
surrounded by orbits (energy shells)