Transcript The History of Atomic Theory

The History of Atomic
Theory
Democritus
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400 BC: Democritus supported
an atomic theory which stated
that “all matter is composed of
atoms”
He said that atoms were
indivisible and indestructible
The word “atomos” comes from
the Greek words for “indivisible”
Some argue that others “said it
first”
http://www.wikipedia.org
John Dalton
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Early 1800s: used
experimental methods to turn
Democritus’ ideas into theory
Dalton’s experiments
involved analyzing the percent
composition of compounds
and their molecular weights
Law of Multiple Proportions:
when elements combine they
do so in a simple whole
number ratio
http://www.wikipedia.org
Described the atom as a small, incompressible sphere with an
atmosphere of heat.
Dalton’s Atomic Theory
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Elements are made of atoms.
All atoms of a given element are identical.
The atoms of a given element are different from
those of any other element
Atoms of one element can combine with atoms
of other elements in whole number ratios to
make compounds
Atoms cannot be created, divided, nor destroyed
in a chemical reaction, just rearranged
J.J. Thomson
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1897: credited with the
discovery of the electron
He was also given credit for
calculating the charge: mass
ratio of the electron
His model of the atom is
known as the plum pudding
model
http://wwwoutreach.phy.cam.ac.uk/camphy/electron/electron
6_1.htm
How did he do it?
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Used famous “Cathode Ray Tube
Experiment”
Scientists prior to Thomson
discovered that the green glow was:
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Coming from the cathode
It had mass, as it could move a
“pinwheel”
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Using the velocity, angle of
deflection, etc., Thomson was able
to make his calculations
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CRT in TV and Magnet
http://www.astro.virginia.edu/class/oc
onnell/astr121/im/crookes-tube.jpg
Robert Millikan
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1909: credited with
experimentally
determining the
charge of an electron
as 1.60 x 10-19 coulomb
Mathematically, he used
Thomson’s charge: mass
ratio to determine the
mass as 9.108 x 10-28 g
www.fordhamprep.org
How did he do it?
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“Millikan’s Oil Drop
Experiment“
With an atomizer, sprayed oil
droplets into box containing
two charged plates
Used X-rays to give droplets
negative charge
As droplets fell between
plates, he measured how
different charges on plates
change the rate of fall
Ernest Rutherford
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1909: Conducts his
famous experiment that
helps him to discover:
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The atom is mostly
empty space
The nucleus
The positively charged
proton
http://www.wikipedia.org
How did he do it?
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"Gold Foil Experiment“
Surrounded thin piece of gold foil with luminescent
screen
“shot” positively charged alpha (α) particles at the foil
Most went straight through, but a very small number
were slightly deflected; an even smaller number were
totally deflected
“It was almost as incredible as if you fired a 15-inch shell at a
piece of tissue paper and it came back and hit you.”
What was happening?
New Zealand Postage Stamp
James Chadwick
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Frederic and Irene JoliotCurie bombarded beryllium
with alpha particles, forming
a powerful beam
1932: Chadwick noted that
the beam was not deflected
by electric or magnetic fields,
thus he determined the beam
had no charge
He predicted that this beam
was composed of neutrally
charged neutrons
www.wikipedia.org
Niels Bohr
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www.wikipedia.org
Studied under Thomson
and Rutherford and built
upon the work of Planck,
Hertz, Einstein, and others
1913: predicted that
electrons are only found in
specific circular paths, or
orbits, around the nucleus
Electrons can only move
from one orbit to another,
but not in between, much
like a ladder
http://knowledgepublications.com/doe/images/DOE_Nuclear_Bo
hr_Model_of_the_Atom.gif
How did he do it?
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Run electricity through gaseous elements, thus
exciting the electrons to a higher energy level
Electrons “fall” back to their ground state and release
the absorbed energy in the form of waves