Transcript J.J. Thomson and the Cathode Ray Tube 1897

History of the Atomic Model
Seeing the Invisible
1
A Big Debate
Can matter be divided into
smaller and smaller pieces
forever?
YES!
NO!
2
Aristotle’s Claim
• The universe consists of the following
– Earth
– Water
– Fire
– Wind
– Void
3
Atomic Models: Greek
Democritus (460-370 B.C.)
 Matter can not be
divided forever
• Smallest piece = “atom”
(Greek “atomos” = “not to be
cut”)
• He had no way of knowing
what atoms looked like!4
The word “atom” comes from a Greek
word that means “unable to be cut”
… and you kept
Imagine
you
on cutting
the
had
a piece
leftover
piece in
of
gold that
half…
you then cut
in half…
…and then you
cut one of these
smaller pieces in
half…
GoGold
ld
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The word “atom” comes from a Greek
word that means “unable to be cut”
…and kept
…and
going…kept
going…
An atom of gold
Eventually you would have 1
…and
keptcut it
piece of gold left.
If you
going… have
in half, you wouldn’t
gold any more – you’d have
something else. This tiny, tiny
single piece of gold is called
an atom of gold. An atom is
the smallest particle of an
element that acts like the
element.
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Democritus
He hypothesized that atoms were:
You can’t really touch atoms. The
– Small & Hard
negative electrons repel you!
– Diff in shape & size
There are only so many atoms in the
– Infinite
universe they MUST be recycled.
Carbon, water, nitrogen cycles, etc.
– Always moving
– Capable of joining
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Who Was Right?
• Greek society was slave based
• No actual experiments
– It was all a thought game
• Settled disagreements by verbal debate
• Aristotle was more famous so he usually
won even if he was wrong.
• His ideas carried through to the middle
ages. (2000 years later)…until the
8
Enlightenment
John Dalton (Late 1700’s)
•
•
•
School teacher in England
Based his conclusions on
experimentation and observations
(Law of Conservation).
Combined ideas of elements & matter
with that of atoms
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Dalton’s Atomic Theory
1. All elements are composed of
submicroscopic indivisible parts called
atoms.
2. Atoms of the same element are identical,
those of different atoms are different.
3. Atoms of different elements combine in
whole number ratios to form compounds.
4. Chemical reactions involve the
rearrangement of atoms. No new atoms
are created or destroyed.
10
Dalton’s Theory
• Most of Dalton’s theory is still accepted today.
• Except the part about atoms being
indivisible…and he was wrong about all
elements of the same type being identical
• These are called isotopes.
• U-238 seen below.
11
Isotopes
• Frederick Soddy (1877-1956)
proposed the idea of isotopes in 1912
• Isotopes are atoms of the same
element having different masses,
due to varying numbers of
neutrons.
• Soddy won the Nobel Prize in
Chemistry in 1921 for his work with
isotopes and radioactive materials.
12
J.J. Thomson
and the Cathode Ray Tube
1897
English physicist
Provided the first evidence that atoms are made
of even smaller particles
Description of a cathode ray tube and a short video of
how it works:
http://www.chem.uiuc.edu/clcwebsite/cathode.html
13
Thomson’s Experiment
-
+
• How it works…
• Cathode/Anode in a vacuum.
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Thomson’s Experiment
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+
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Thomson’s Experiment
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+
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Thomson’s Experiment
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+
• Passing an electric current makes a beam appear to
move from the negative to the positive end.
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Thomson’s Experiment
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+
• Passing an electric current makes a beam appear to
move from the negative to the positive end.
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Thomson’s Experiment
• By adding an electric or magnetic field
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Thompson’s Experiment
• By adding an electric or magnetic field
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Thompson’s Experiment
• By adding an electric or magnetic field
21
Thompson’s Experiment
• By changing the electric field he found the
moving particles were negative
• This is still how all CRT TV’s work.
• Don’t try this at home! MAGNET LINK
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Thompson’s Model
• Found the electron
• Concluded that there must be
a small positive charge
everywhere since atom was
neutral
• Atom was like plum pudding
– A bunch of positive stuff, with
electrons able to be removed (as
in his experiment, static, etc.)
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Ernest Rutherford
• Former student of J.J.
Thomson
– Believed in plum pudding model
1871-1937
• Wanted to find out how big
they (electrons) are
• Fired positively charged
radiation called alpha particles
at a piece of gold foil, which
can be made a few atoms thick
24
Rutherford’s Experiment
• When alpha particles hit a fluorescent
screen it will glow.
• Here’s what the setup looked like …
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• What he expected to see: No change in the
path..
• This would have confirmed his atomic model
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– Rutherford thought alpha particles should
pass through without a change in direction
– Positive charges in this model are spread
out evenly. Alone they should not be
enough to stop an alpha particle
Wrong!
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• He was wrong… Rutherford animation
http://micro.magnet.fsu.edu/electromag/java/rutherford/
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• How he explained it
1. Atom is mostly empty
2. Small dense, positive piece at the center:
he found the nucleus!
3. Alpha particles are deflected if they get
close enough to positive center
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Niels Bohr (1885-1862)
• Electrons have orbits about the
nucleus (planetary model)
• Electrons could only exist at
given energy levels
• An energy level is where an
electron is likely to be moving
• Energy levels were like steps
on a ladder
– An electron can only be at any
given step at any given time
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De Broglie and Schroedingers (1924-25
Wave Model AKA
Quantum Mechanical Model
The Wave Model a.k.a. The
Quantum Mechanical Model
• Today’s atomic
model is based on
the principles of
wave mechanics.
• Electrons behave
almost like light. do
not move about an
atom in a definite
path, like the
planets around the
sun.
The Wave Model a.k.a. The
Quantum Mechanical Model
(this is the closest to reality but we can’t really use it precisely…)
• In fact, it is impossible to determine the exact location of an
electron. The probable location of an electron is based on
how much energy the electron has.
• According to the modern atomic model, at atom still has
small positively charged nucleus surrounded by a large
electron cloud region in which there are enough electrons
to make an atom neutral.
• This cloud is really Most of the volume of an atom
• Cloud is a ‘Probability region’ where electron may be found
at any given time.
• This structure only possible if electrons behave both like
electromagnetic waves (light) and matter and are both ..until
we observe them then they ‘become’ one or other
• Schrödinger …no likey.
Schrödinger's Cat Confusion
Schrödinger thought it was dumb to have two states
wave/matter and DISAGREED with the idea of both being
simultaneous possible. No really!
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Isotopes
• Atoms of the same element can have
different numbers of neutrons
• Different mass numbers called isotopes
• For example H1 H2 H3 (all have one
proton and are hydrogen with only one
proton but are also called protium
deuterium, tritium)
• Hydrogen was responsible for the
explosion in Japans Nuclear Power
Plant (not a nuclear explosion!). Still
issues today
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Atomic
Models
Indivisibl
e
Democritus
X
Dalton
X
Electron
Nucleus/Proton Orbit
Thomson
X
Rutherford
X
X
Bohr
X
X
Schroedinger
X
X
DeBroglie
Wave or Quantum
Mechanical Model
Electron
Cloud
‘orbital’
X
X