The document summarizes the development of the atomic theory from ancient Greek philosophers like Democritus, who proposed atoms as indivisible particles, to modern scientists like Rutherford, Thomson, Millikan, and Chadwick. Key events included Dalton proposing atoms combine in whole number ratios (1803), Thomson discovering the electron (1897), Rutherford deducing the nuclear atom from gold foil experiments (1911), and Chadwick discovering the neutron (1932). Together, these scientists experimentally verified atoms and identified the subatomic particles that compose them.
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Chpt 4 atomic theory 101404
1. The Origins
of the
Atomic Theory
Pisgah High School
Rev. 1/30/02
M . Jones 1/5/04
10/14/04
2. The Development of the
Atomic Theory
• Democritus and Dalton: atomic theory
• Crookes, Roentgen, Becquerel, Curies:
early evidence for subatomic particles
• Thomson: CRT’s and the electron
• Millikan: “Oil Drop Experiment”
• Rutherford: “Gold Foil Experiment”
• Chadwick: Neutron
3. Democritus
• Greek philosopher ~ 300 BC
• Limit to “smallness”
• All matter consists of tiny,
indestructible particles called
atoms
• Atomos – indestructible
• Aristotle and infinity
4. John Dalton
First serious atomic theory
• English scientist
• Studied the properties of gases
• “Reinvented” the idea of atoms
• Published in 1803
5. Dalton’s atomic theory - 1803
1. Elements are composed
of tiny, discrete, particles
called atoms.
6. Dalton’s atomic theory - 1803
2. Atoms are indivisible
and indestructible and do
not change their identity
during reactions.
7. Dalton’s atomic theory - 1803
3. Atoms of the same
element are identical in
mass and chemical and
physical properties. Atoms
of different elements are
different.
8. Dalton’s atomic theory - 1803
4. Atoms combine to form
compounds in simple,
whole-number ratios.
Law of Definite Proportions
9. Dalton’s atomic theory - 1803
5. Atoms combine in
different ratios to make
two or more compounds.
Law of Multiple Proportions
10. Dalton’s Atomic Theory
1. Atoms are tiny, discrete particles
2. Atoms are indestructible
3. Atoms of the same element have the
same mass and properties
4. Atoms combine in simple whole-
number ratios
5. Atoms in different ratios produce
different compounds.
11. Dalton’s Atomic Theory
1. Atoms are tiny, discrete particles
2. Atoms are indestructible
3. Atoms of the same element have the
same mass and properties
4. Atoms combine in simple whole-
number ratios
5. Atoms in different ratios produce
different compounds.
12. Evidence for subatomic particles
During the 19th century many
discoveries were made that were later
shown to involve subatomic particles.
Cathode rays, canal rays, X-rays,
and then alpha, beta and gamma
rays were discovered and studied.
13. Evidence for subatomic particles
People like Crookes, Goldstein,
Roentgen, Becquerel, Pierre and Marie
Currie, Thomson, Millikan, Rutherford
and others all paved the way for us to be
able to talk about atoms.
Each helped advance the foundation
of what would later become the
atomic theory.
14. William Crookes
Studied spectroscopy and
discovered thallium.
Used vacuums to measure the mass.
Invented the radiometer.
Made better vacuums.
Techniques which were used by
Edison to make light bulbs.
15. William Crookes
Developed
what was
called the
Crookes’ Tube
… which is what we
now call a cathode ray tube.
16. William Crookes
Used the cathode ray tube to to
study electric fields in vacuum and
discovered rays, …
which were called “cathode rays”
by Goldstein, since they came
from the cathode, or negative
electrode.
18. William Crookes
He found that the cathode
rays could be deflected by
a magnet.
This suggested that the
cathode rays might be a
stream of charged particles.
19. Cathode Ray Tube
Direction of
cathode rays
Cathode Anode
High voltage +
20. Cathode Ray Tube
Magnet
Direction of
cathode rays
Cathode Anode
High voltage +
21. Wilhelm Roentgen
Used cathode rays to study the
luminescence the rays created in
certain chemicals.
To observe the faint glow, he
surrounded the cathode ray
tube with black cardboard.
22. Wilhelm Roentgen
Discovered that some barium
platinocyanide was glowing
even though none of the
cathode rays could reach it
because they were blocked by
the cardboard.
23. Wilhelm Roentgen
An invisible radiation was coming
from the cathode ray tube and
passing through the cardboard.
He called them X-rays.
Now we know X-rays as high energy
electromagnetic radiation caused by
the sudden stopping of electrons.
24. Henri Becquerel
Wanted to see if fluorescent
substances produced X-rays.
Out in the sun, he put a crystal of a
fluorescent chemical on
photographic film which was
wrapped in black paper.
The chemical was uranium sulfate.
26. Henri Becquerel
After several cloudy days with the
uranium sulfate and wrapped film
safely in a drawer,
he processed
the film to see
if there was
any residual
fluorescence.
27. Henri Becquerel
This was what he found.
The fogged area was even larger.
The fogging
did not
involve either
sunlight or
fluorescence.
28. Henri Becquerel
He studied the radiation
from the uranium Found it similar
compound. to X-rays.
Could
penetrate
materials and
ionize air.
29. Henri Becquerel
The radiation was not X-rays.
It was a new kind of radiation,
But it behaved from a new source.
like X-rays.
Marie Curie
named it
radioactivity.
30. Henri Becquerel
He also found that radioactivity
could be deflected by a magnet.
Could be steams of tiny charged
particles.
In 1900 he
decided they
were electrons.
31. Three kinds of radioactivity
• Alpha particles − α
• Beta particles − β
• Gamma rays − γ
These were named by
Ernest Rutherford.
32. Three kinds of radioactivity
• Alpha particles - helium nuclei
• Beta particles - electrons
• Gamma rays - high energy
electromagnetic
energy
33. Radioactivity …
… the natural decay of unstable atoms.
… can be detected by photographic
film or a Geiger counter.
… is “ionizing radiation”. Causes cells
damage and mutations – cancer.
… is protected against by shielding and
distance.
36. Alpha Particles
Helium nucleus (2 p + 2 n).
Relatively massive and slow.
Very dangerous when inside the
body. 100% absorbed.
Blocked by 2.5 cm of air, 3-4
sheets of paper or by skin.
37. Beta Particles
Electrons that come from the decay
of neutrons in the nucleus
Much less massive.
Much faster than alphas.
Dangerous to cells.
Blocked by metals or plastic.
38. Gamma Rays
Electromagnetic energy, not
particles.
Like light but invisible, much higher
energy and shorter wavelengths.
Travel at the speed of light.
Have no mass.
39. Gamma Rays
Can easily pass through your body,
and can damage cells.
Greater penetrating power.
Blocked by many inches of lead
or many feet of concrete.
41. Alpha, Beta, Gamma
Paper Lead
α
Aluminum foil
Radioactive or wood
Source
42. Alpha, Beta, Gamma
Paper Lead
β
α
Aluminum foil
Radioactive or wood
Source
43. Alpha, Beta, Gamma
Paper Lead
β
γ
α
Aluminum foil
Radioactive or wood
Source
44. Cathode Ray Tube
It was also used by J. J. Thomson
Cathode Anode
High voltage +
45. J. J. Thomson
• Cathode rays - cathode ray tube
• Attracted to positive electrode
• Thought they might be atoms
• Had same charge to mass ratio
regardless of metal in the cathode
• Particle must be common to all
matter, a subatomic particle
46. That particle was
called the …
The
The electron electron
The Electron
The electron
The electron
Discovered in 1897
The electron
By J. J. Thompson
47. The term “electron” actually comes
from George Stoney’s term for the
“minimum electrical charge”.
After the discovery of the electron,
it was assumed that this particle
was the carrier of the minimum
electrical charge and so the particle
was called an “electron”.
48. J. J. Thomson
Even though Crookes and others
observed and characterized
cathode rays, Thomson is
credited with the discovery of the
electron because he recognized
that it was a fundamental particle
of nature -- a sub-atomic particle.
49. J. J. Thomson
Measured the charge to mass
ratio, and found …
… that if this “minimum charge”
was equal to the charge on a
hydrogen ion, then the mass of
the electron would be /1837th the
1
mass of a hydrogen atom.
50. J. J. Thomson
If that were the case, then the
electron would be much smaller
than the smallest atom,
… showing for the first time
that matter is made up of
particles smaller than atoms.
Thomson tried to measure the
fundamental charge on the electron.
51. Robert A. Millikan
Robert A. Millikan, an American
physicist, set out to determine the
charge on an electron.
From 1909 through 1910, he
performed what is now called
the “Oil Drop Experiment”.
52. Robert A. Millikan
Radiation stripped electrons from the
Atomizer
oil droplets. The charged droplets fell
between two electrically charged
plates. By adjusting the voltage, he
Oil Drop
High
could change the rate of fall or rise of
Voltage Telescope
a single oil drop. After observing
Cast iron pot
hundreds of drops,iron pot
Cast he calculated the
charge on a single electron.
53. Robert A. Millikan
Atomizer
High Oil Drop
Voltage Telescope
Cast iron pot
55. Robert A. Millikan
The fundamental charge on an
electron is 1.602 x 10-19 coulombs.
With J. J. Thomson’s charge to mass
ratio, and Millikan’s charge on the
electron, we are able to compute the
mass of an electron:
9.1 x 10-28 gram
56. Ernest Rutherford
• Authority on radioactivity.
• Named alpha, beta and gamma rays.
• Geiger and Marsden do a series of
alpha scattering experiments. (1909)
• Most alpha particles undeflected.
Few underwent large changes –
some came back toward source.
• Similar to shooting at tissue paper
62. Gold Foil Experiment: Review
α source
The positive α
+ particles are
repelled by the
nucleus.
63. Rutherford’s Nuclear Atom
Alpha particles were repelled by…
… a small, dense, positively
charged nucleus.
Almost all the mass of an
atom is in the nucleus.
Electrons are located outside the
nucleus. Published results in 1911.
64. Rutherford and the Proton
1917 – 1924: Rutherford experimented
with radioactivity and protons.
Bombarded the lighter elements with
alpha particles. Some protons were
knocked loose - transmutation occurred.
The first person to cause a change from
one element to another.
N+α O+H
65. Rutherford and the Proton
N+α O+H
7 protons 1 proton
2 protons 8 protons
9 protons
9 protons
66. Chadwick and the Neutron
Worked with Rutherford on alpha
bombardment from 1919.
Then later on the search for a
neutral particle in the nucleus.
Both disagreed with the current
theory of extra protons and electrons
in the nucleus.
67. Chadwick and the Neutron
Particles can be detected by their
ability to ionize air, but neutral
particles did not ionize air.
He repeated experiments (1932)
which showed an undetected
radiation knocking protons out of
paraffin.
The radiation consisted of neutrons.
68. Many more scientists
contributed to the
development and
refinement of the
atomic theory.
