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Early History of Atomic Theory
Early History of Atomic Theory
The idea that matter was made
up of small particles is not new.
In ancient Greece, it was thought
that matter was composed of four
elements: earth, air, water, & fire.
At the time, there were two competing theories
about the composition of matter:
1)The continuous theory of matter and ...
2)The discontinuous theory of matter.
Continuous Theory of Matter
The continuous theory of matter said that
matter is made of a material that could
forever be cut in half without end.
▪
You could keep on going
forever and ever!
Discontinuous Theory of Matter
The discontinuous theory of matter said that
matter is made up of basic particles that could
not be cut in half.
Eventually, an “uncutable”
piece of matter is found!
Early History of Atomic Theory
The word atom comes from the Greek
“atomos” which means “uncutable”.
The continuous theory
of matter was supported
by the famous Greek
philosopher Aristotle
and as a result the
discontinuous theory of
matter fell from favor and
was not revived for 2000 years.
Early History of Atomic Theory
 In 1803, John Dalton first
proposed a modern atomic
theory which included the
following propositions:
1) Elements are composed
of atoms that are small
hard indivisible particles.
(Dalton proposed that atoms
were small, solid, indivisible,
and indestructible particles
of different sizes and shapes).
Early History of Atomic Theory
2) Atoms of the same element are exactly
the same ...
... and atoms of different atoms are
different.
Early History of Atomic Theory
3) Atoms can NOT be created, destroyed,
divided nor changed in any way.
Atoms will exist UNCHANGED forever.
Early History of Atomic Theory
4) Compounds are composed of atoms of
different elements in definite ratios.
Early History of Atomic Theory
5) Chemical reactions occur when the atoms
rearrange themselves to make new groups.
Early History of Atomic Theory
According to Dalton’s theory, atoms are the
basic building blocks of matter.
Dalton’s theory CAN explain how elements
and compounds have an exact composition.
Dalton’s theory can NOT explain ...
... what atoms are made of or ...
... how atoms are structured (put together).
It would take over ninety years before some
of these questions could be answered.
Early History of Atomic Theory
In 1897, JJ Thomson introduced a new
atomic theory as a result of experiments
using cathode ray tubes.
Early History of Atomic Theory
A cathode ray tube
is a glass tube with
the air removed.
Cathode (-)
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Early History of Atomic Theory
When a high voltage was applied, a greenish
glow developed at the end of the tube which
had been coated on the inside with a special
substance.
Cathode (-)
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Early History of Atomic Theory
• It was proposed that an unusual invisible
beam of light was emitted from the cathode
which then traveled across the tube and
caused the special substance to glow green.
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Early History of Atomic Theory
JJ Thomson placed an object in the middle
of the tube, which then cast a very sharp
"shadow" at the end of the tube.
Normal light would have cast a fuzzy
"shadow".
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Early History of Atomic Theory
J.J. Thomson devised some experiments in
which he used a variety of gases in the tube
and a variety of materials for the cathode.
The results were the always same.
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Early History of Atomic Theory
He then placed a barrier with a small opening
in front of the cathode.
This helped to show that the cathode rays
traveled in a straight beam.
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Early History of Atomic Theory
He then passed the beam of cathode rays
between oppositely charged electric plates.
The beam was always deflected toward the
positive plate.
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Early History of Atomic Theory
This showed that cathode rays had a negative
charge.
Thomson concluded that cathode rays
consisted of negatively charged particles
which today we call electrons!
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Early History of Atomic Theory
Thomson proposed that an atom was a
positively charged solid that was studded
with negatively charged sub-particles which
we now call electrons. His model is often
referred to as the “plum pudding” model.
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Early History of Atomic Theory
In 1908, Ernest Rutherford conducted a famous
experiment that utterly changed atomic theory.
Early History of Atomic Theory
He bombarded super thin gold foil with alpha
() rays - positively charged particles.
Alpha
ray (+)
emitter
-particles
super thin
gold foil
The inside of the
screen was coated
with a fluorescent
material which
sparked when an
alpha particle hit it.
Early History of Atomic Theory
Over 99% of the alpha particles passed right
straight through the gold foil.
Alpha
ray (+)
emitter
-particles
super thin
gold foil
Over 99% of the
small sparks were
observed here.
Early History of Atomic Theory
Less than 1% of the alpha particles were
deflected from their straight-line course as
they passed through the gold foil.
Less than 1% of the
small sparks were
observed here.
Alpha
ray (+)
emitter
-particles
super thin
gold foil
Less than 1% of
the small sparks
were observed here.
Early History of Atomic Theory
To their amazement, about 1 alpha particle in
8,000 to 10,000 seemed to bounce back from
the gold foil.
1 in 8000 to 10,000 of
the small sparks
were observed here.
Alpha
ray (+)
emitter
-particles
super thin
gold foil
Early History of Atomic Theory
This is what Rutherford theorized.
Gold foil is in
reality, around
8500 atoms thick
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Deflected alpha
particle passed very
close to a positively
charged nucleus.
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Alpha
particles
approaching
the gold foil
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Reflected alpha particle collided
with a super dense nucleus.
Unaffected alpha particles
passed right through the
atoms as if nothing was
there (empty space)!
Early History of Atomic Theory
Rutherford’s atomic theory suggested that an
atom’s volume is almost 100% empty space.
It has a small nucleus that is
1/10000th the volume of the
atom, but which has almost
.
100% of the atom’s mass.
The nucleus has a positive
charge due to the presence
of protons. The electrons were
thought be at the outer edge of the atom.
Rutherford’s model is often called “the empty
space model”.
Early History of Atomic Theory
The three early models of the atoms are ...
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Dalton’s
Model:
A small hard
particle made
of a solid
material
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Thomson’s
Model:
A small hard
particle made of
a solid positively
charged material
that has negative
electrons in it.
Rutherford’s
Model:
A small empty
particle with a
dense positively
charged nucleus
and with negative
electrons on the
outer edge