chpt 4 atomic theory 101404
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The Origins The Origins of the of the
Atomic TheoryAtomic TheoryPisgah High SchoolPisgah High School
M . JonesM . Jones Rev. 1/30/021/5/04
10/14/04
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
Democritus• Greek philosopher ~ 300 BC• Limit to “smallness”• All matter consists of tiny,
indestructible particles called atoms
• Atomos – indestructible• Aristotle and infinity
John DaltonFirst serious atomic theory
• English scientist
• Studied the properties of gases
• “Reinvented” the idea of atoms
• Published in 1803
1. Elements are composed of tiny, discrete, particles
called atoms.
Dalton’s atomic theory - 1803
2. Atoms are indivisible and indestructible and do not change their identity
during reactions.
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.
Dalton’s atomic theory - 1803
4. Atoms combine to form compounds in simple, whole-number ratios.
Dalton’s atomic theory - 1803
Law of Definite Proportions
5. Atoms combine in different ratios to make
two or more compounds.
Dalton’s atomic theory - 1803
Law of Multiple Proportions
Dalton’s Atomic Theory1. Atoms are tiny, discrete particles 2. Atoms are indestructible3. Atoms of the same element have the
same mass and properties4. Atoms combine in simple whole-
number ratios5. Atoms in different ratios produce
different compounds.
Dalton’s Atomic Theory1. Atoms are tiny, discrete particles 2. Atoms are indestructible3. Atoms of the same element have the
same mass and properties4. Atoms combine in simple whole-
number ratios5. Atoms in different ratios produce
different compounds.
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.
Evidence for subatomic particles
Each helped advance the foundation of what would later become the atomic theory.
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.
William CrookesStudied 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.
William Crookes
Developed Developed what was what was called the called the
Crookes’ TubeCrookes’ Tube
… … which is what we which is what we now call a cathode ray tube.now call a cathode ray tube.
William CrookesUsed 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.
Cathode Ray Tube
High voltageHigh voltage
Cathode Anode
Direction of cathode rays
Near-vacuum inside the glass tube
+
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.
Cathode Ray Tube
High voltage
Cathode Anode
Direction of cathode rays
+
Cathode Ray Tube
High voltageHigh voltage
Cathode Anode
Direction of cathode rays
+
Magnet
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.
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.
Wilhelm RoentgenAn invisible radiation was coming from the cathode ray tube and passing through the cardboard.
Now we know X-rays as high energy electromagnetic radiation caused by the sudden stopping of electrons.
He called them X-rays.
Henri BecquerelWanted 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.
Henri Becquerel
Radiation penetrated the black paper.
Must be X-rays.
Because, when developed, the film was fogged.
Henri BecquerelAfter 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.
Henri Becquerel
This was what he found.
The fogged area was even larger.
The fogging did not involve either sunlight or fluorescence.
Henri BecquerelHe studied the radiation from the uranium compound.
Found it similar to X-rays.
Could penetrate
materials and ionize air.
Henri Becquerel
Marie Curie named it
radioactivity.
The radiation was not X-rays.
It was a new kind of radiation, from a new source.But it behaved
like X-rays.
Henri BecquerelHe also found that radioactivity could be deflected by a magnet.
Could be steams of tiny charged particles.
In 1900 he decided they were electrons.
Three kinds of radioactivity
These were named by Ernest Rutherford.
• Alpha particles
• Beta particles • Gamma rays
Three kinds of radioactivity
- helium nuclei
- electrons
- high energy electromagnetic energy
• Alpha particles
• Beta particles • Gamma rays
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.
Properties of Radiation
Alpha, Beta and Gamma
Look at shielding, speed, hazards and
mass.
Properties of Radiation
Alpha, Beta and Gamma
Identity
Relative Mass
Relative Speed
Hazards
Shielding
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.
Alpha 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.
Beta Particles
Electromagnetic energy, not particles.
Like light but invisible, much higher energy and shorter wavelengths.
Travel at the speed of light.Have no mass.
Gamma Rays
Can easily pass through your body, and can damage cells.
Gamma Rays
Greater penetrating power. Blocked by many inches of lead or many feet of concrete.
Alpha, Beta, Gamma
Radioactive Source
- - - - - - - - -
+ + + + + + + +
Electrically charged plates
Alpha, Beta, Gamma
Radioactive Source
Paper Lead
Aluminum foilor wood
Alpha, Beta, Gamma
Radioactive Source
Paper
Aluminum foilor wood
Lead
Alpha, Beta, Gamma
Radioactive Source
Paper Lead
Aluminum foilor wood
Cathode Ray Tube
High voltage
Cathode Anode
+
It was also used by J. J. Thomson
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
That particle was called the …
The electron
The electronThe electronThe electron
The electron
The ElectronDiscovered in 1897By J. J. Thompson
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”.
J. J. ThomsonEven 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.
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 1/1837th the
mass of a hydrogen atom.
J. J. ThomsonIf 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.
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”.
HighVoltage
Cast iron pot
Robert A. Millikan
Telescope
Atomizer
Oil Drop
Cast iron pot
Radiation stripped electrons from the oil droplets. The charged droplets fell between two electrically charged plates. By adjusting the voltage, he could change the rate of fall or rise of a single oil drop. After observing hundreds of drops, he calculated the charge on a single electron.
HighVoltage
Robert A. Millikan
Telescope
Cast iron pot
Atomizer
Oil Drop
Robert A. Millikan
Charges on drops are multiples of 1.602 x 10-19 coulombs.
Robert A. MillikanThe 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
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
The Gold Foil Experiment
Top View
Side View
The Gold Foil Experiment
Alpha particle source
Gold foil Fluorescentdetector
ZnS
All of this was in a vacuum chamber.
The Gold Foil ExperimentMost of the
particles went…
…straight through the gold foil, undeflected.
The gold is mostly “empty space.”
Alpha Particles
Alpha particles are helium nuclei.
++
Two protons and
two neutrons.
The alpha particle is positively charged.
Gold Foil Experiment: Results
+
source
Small, dense, positively charged
nucleus of gold
Gold Foil Experiment: Review
+
source
The positive particles are
repelled by the nucleus.
Rutherford’s Nuclear AtomAlpha 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.
Rutherford and the Proton1917 – 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 + O + H
Rutherford and the Proton
N + O + H
7 protons
2 protons
1 proton
8 protons
9 protons 9 protons
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.
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.
Many more scientists contributed to the development and refinement of the
atomic theory.
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