Transcript Thank you Sir William Crookes!!!!! For your Accidental

Atomic Theory
Chapter 8
The Composition of the atom
John Dalton
• 1800 -Dalton proposed a modern atomic model
based on experimentation not on pure reason.
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All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine
in constant ratios to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the law of conservation of
mass (atoms are neither created nor destroyed)
and the law of constant composition (elements
combine in fixed ratios).
1803 John Dalton
• Dalton’s Model of the
Atom:
– Uniform Shape
– Uniform Density
– Indivisible
• So what did Dalton’s
model look like?
• A simple design that
promoted future research
into atomic theory…
Discovering the Electron
• He noticed a flash of light within
one of the tubes.
• Flash was produced by some
form of radiation striking the light
at the end of the tube.
• There were rays (radiation)
traveling from the cathode to the
anode in the tube.
• They called it the cathode ray b/c
the ray of radiation originated
from the cathode end of the tube.
• Thank you Sir William Crookes!!!!!
The Cathode Ray Tube (CRT)
Developed by Sir William Crookes in
the late 1800s
Thank you Sir William Crookes!!!!!
For your Accidental Discovery!!!!! 1879
• Discovery of
cathode rays led
to invention of the
TV!!!!
•TV and computer
monitor images are
formed as radiation from
the cathode strikes the
light producing
chemicals that coat the
backside of the screen.
JJ Thompson 1897
• Using the CRT…
Cathode
(--)
Anode (+)
Electromagnets
(turned off)
Beam of “light”
Fluorescent Screen
JJ Thompson 1897
• When the magnets were turned on, the
beam was pulled towards the + plate
• What could Thompson conclude?
+ side
-- side
JJ Thompson 1897
• What did his experiment discover?
• ELECTRONS! (negatively-charged particles)
• How did the model of the atom change?
AKA The Chocolate-Chip Cookie
Model
Robert Millikan 1909
• Determined the
charge of an
electron.
• Mass of an electron
= 9.1 x 10-28 g
• Mass of an electron
is extremely small.
Millikan’s Experiment
• Measured the time it took for an
electrically charged oil drop to rise a
measured distance when subjected to
an electric field
• Measured the time it took electrically
charged droplets to fall with no electric
field
Oil Drop Experiment
Millikan’s Outcomes
• Determined the charge on a single
electron
• Using the charge from Millikan and
the charge-to-mass ratio from
Thomson, many scientists
determined the mass of an electron.
Discoveries from the CRT
• Wilhelm Roentgen
(1895)
?
●Discovered x-rays
●Killed his wife
Henri Becquerel 1896
• Roentgen’s Discovery of X-rays provoked
further research by many others, including
Becquerel
• Since fluorescent materials could be
charged by X-rays, Henri wondered if
naturally fluorescent minerals could give
off X-rays….
• Like any good scientist, he set up a test…
Henri Becquerel 1896
Photographic film
sealed away
Henri Becquerel 1896
The photographic
paper was still
exposed!
The mineral contained
Uranium!
Serendipity- accidental discovery
Becquerel’s Outcomes
● Left
a sample of uranium ore in a drawer
with photo plates and there was fogging on
the plates
• Concluded something in the sample
itself must be capable of “fogging” the
plates; the sample must give off some
kind of ray without the influence of the
sun
• Rays were the products of radioactivity
• 1896 - credited with the discovery of
radioactivity
Marie and Pierre Curie
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French Scientists
Intrigued with Becquerel’s work
Studied radioactivity
Marie discovered the element POLONIUM
Ernst Rutherford 1898
• After the discoveries of Becquerel and
Thompson, Rutherford was dissatisfied
with the atomic models and set out to
disprove it.
• Using Uranium, he and his lab assistants
(Geiger and Marsden) set out…
Ernest Rutherford 1911
• Studied how positively
charged alpha particles
interacted with solid
matter.
Conducted an
experiment to see if
alpha particles would be
deflected as they passed
through a thin foil of
gold. Also known as
Rutherford’s Gold Foil
Experiment.
Ernest Rutherford’s Gold Foil Experiment
1. A narrow beam of alpha
particles was aimed
at a thin sheet of
gold foil.
A few times they
were deflected at
very large angles.
2. A zinc sulfide coated screen
surrounding the gold foil
produced a flash of light
whenever it was struck by an
alpha particle.
1. Most all of the
alpha particles
passed straight
through the gold
foil, without
deflection.
2. Some alpha
particles were
scattered at small
angles.
Rutherford’s Gold Foil Conclusions
1. Rutherford said plum
pudding model was
incorrect.
2. An atom consisted mostly
of empty space through
which the electrons move.
3. There was a tiny, dense
region, called the
nucleus, centrally
located within the atom that
contained all of an atom’s
positive charge and almost
all of its’ mass.
Planetary Motion Model
• Electrons orbit around
the nucleus like
planets orbit around
the sun
• The nucleus held only
protons (now)
Hans Geiger
• Student of Rutherford
• Spent long hours counting the particles
that hit the screen
• Created the first electric particle counter
- The GEIGER counter
• Determined that alpha particles had 2 p+
and 2n0 (1908)
Irene Curie-Joliot & Frederic
Joliot
• Bombarded Be with alpha particles formed a beam of energy with great power
which was able to penetrate metals
• Thought it was new form of energy thought it was gamma rays
James Chadwick
• Found new beam of energy was unaffected
by magnetic fields - so it was neutral
• Particles moved at 1/10 the speed of light so it was NOT radiant energy - but must be
particles
• The new particles had approximately the
same mass as a proton - but they had no
charge
• Credited with the discovery of the
NEUTRON
After Chadwick
• After neutrons were discovered - isotopes
of various elements were discovered
• We will come back and pick up our story in
a little bit.