A History of the Atomic Model

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A History of the Atomic Model

• Sarah L. Wood

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"Men need ideas to live, but so do ideas
need men to live. The history of science, like
all history, is the record, and our judgment of
the record, of res gestae the deeds of fallible
men. Greenaway, 1966, pg. 4
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Five Models

• The solid ball (440 BCE-1904 CE)
• The plum pudding model (1904-1911)
• The nuclear model (1911-1913)
• The orbital atom (1913-1930)
• The electron cloud model (1926-present)

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Why study the atom?

• Its the building block of all that we know!
• nuclear reactions formed stars and all the
  elements

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Why study the atom?

• Its the building block of all that we know!
• nuclear reactions formed stars and all the
  elements

• Applications
• Radiation therapy
• X-ray technology
• Atomic bomb
• Nuclear energy

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Why study the atom?

• Its the building block of all that we know!
• nuclear reactions formed stars and all the
  elements
• Applications
• Radiation therapy
• X-ray technology
• Atomic bomb
• Nuclear energy
• Great example of scientific progression in
  general
• Thomas Kuhn, The Structure of Scientific
  Revolutions
• Paradigm established
• Paradigm generally accepted by scientific
  community
• Research advances efficiently by conforming to
  paradigm
• Anomolies accumulate making conformation
  impossible
• Anomolies are often discovered because of new
  instruments or methods
• Paradigm modified or replaced

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"The idea of an atom seems so clear if one says
the word 'atom' quickly without thought, that one
is hardly ever aware of its vagueness.
Greenaway, 1966
8
Solid Sphere

• 440 BCE 1904 CE
• John Dalton, officially
• Represented an indivisible particle
• No components
• Not electrically charged

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The Greeks

• Philosophical vs. experimental approach
• Formed hypothesis which explain observations
• Process of natural science was initiated

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The Greeks

• Leucippus and student Democritus of Abdera
• 440 BCE
• Wanted to reconcile permanent, unchanging reality
  with immediate, varying existence
• Eventually there is something indivisible
• Atomos a not, tomos cut
• Multiple shapes, sizes, colors

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The Greeks

• Atoms lay and interact in a void
• Trouble
• Existence of a void was constantly debated
• Aristotle opposed atomism mostly due to the void
  concept

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Corpuscularian Doctrine

• Teachings of Aristotle considered divine
• In 1624, Parliament of Paris equated atomism with
  atheism
• In 1649, Pierre Gassendi declared atoms Dei
  gratia as a gift of God
• He also heavily quoted Torricellis 1643 mercury
  barometer experiment with genuine vacuum

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Corpuscularian Doctrine

• Gassendi called union of atoms to form groups
  moleculae or corpuscula
• Held together with mechanical forces like
  hooks-and-eyes or antlers
• Galileo may have first introduced the idea of the
  corpuscle, strengthening the theme of atomism in
  the 17th century
• Robert Boyle thought that corpuscles clustered to
  account for properties like redness, sweetness,
  hardness, etc.
• Invention of the microscope lent credibility to
  corpuscularian doctrine because of its matter and
  motion account of reality.

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The Chemical Revolution (late 18th century)

• Antoine-Laurent Lavoisier
• Defined a chemical element
• Laboratory analyses to test and prove
• Formula for conservation of matter
• Differentiated element and compound

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The Chemical Revolution(early 19th century)

• John Dalton
• Connected atoms to elements
• Found relative weights of each atom
• It is one great object of this work, to shew the
  importance and advantage of ascertaining the
  relative weights of the ultimate particles.
• Greenaway, 1966, pg. 132

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John Daltons Atomic Theory

• Chemical elements are made of atoms
• Atoms of an element are identical in their masses
• Atoms of different elements have different masses
• Atoms only combine in small, whole number ratios
• Forerunners
• Law of Constant Composition
• Law of Multiple Proportions
• Law of Reciprocal Proportions aka Law of
  Equivalents

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John Dalton
"We might as well attempt to introduce a new
planet into the solar system, or to annihilate
one already in existence, as to create or destroy
a particle of hydrogen. All the changes we can
produce consist in separating particles that are
in a state of cohesion or combination, and
joining those that were previously at a
distance. Greenaway, 1966, pg. 132
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Joseph John (J. J.) Thomson

• April 30, 1897
• Cathode rays are negatively charged particles
• Discovery of the electron
• Much lighter than anything known
• Negatively charged
• Proposed plum pudding model in 1904

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Plum Pudding Model

• 1904 1911
• Joseph John Thomson
• Had to be neutral overall
• Had to possess negative electrons
• Had to have mass Dalton found somewhere other
  than electrons
• Sopositively charged sphere with negatively
  charged electrons embedded

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Ernest Rutherford

• Student of Thomsons at Cavendish Lab
• Moved to Manchester University
• Hans Geiger and Ernest Marsden
• Worked on scattering of alpha particles
• Fired into gold foil
• A few were deflected at nearly ninety degrees and
  occasionally one bounced straight back!

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Ernest Rutherford

• It was quite the most incredible event that
  has ever happened to me in my life. 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.
• Andrade, 1964, pg. 111

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Ernest Rutherford

• Conclusions in 1911
• Atom is mostly empty space
• very, very small, heavy, highly charged
  particlesurrounded by a sphere of
  electrification, very thinly spread, of opposite
  charge
• Called Small, heavy, highly charged particle a
  nucleus

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Nuclear Model

• 1911 1913
• Ernest Rutherford
• Mostly empty space
• Nucleus in center (positive charge)
• Electrons orbited nucleus like planets
• orbit the sun (negative charge)

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Max Planck

• Characterization of light
• Electromagnetic radiation is not continuous
• Emitted in packets of energy quanta
• Photon light quantum
• Size of quanta frequency x h
• Explains line spectra

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Albert Einstein

• Even the Greeks had already conceived the
  atomistic nature of matter and the concept was
  raised to a high degree of probability by the
  scientists of the nineteenth century. But it was
  Planck's law of radiation that yielded the first
  exact determination - independent of other
  assumptions - of the absolute magnitudes of
  atoms.

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More than that, he showed convincingly that in
addition to the atomistic structure of matter
there is a kind of atomistic structure to energy,
governed by the universal constant h, which was
introduced by Planck. This discovery became the
basis of all twentieth-century research in
physics and has almost entirely conditioned its
development ever since. Without this discovery it
would not have been possible to establish a
workable theory of molecules and atoms and the
energy processes that govern their
transformations. Haselhurst,
2006, par. 8
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Niels Bohr

• Rutherfords friend
• Studied line spectrum of hydrogen
• Nuclear model doesnt work
• Needs help of quantum mechanics
• Reasoning
• Electrons orbit the nucleus
• Classical physics tells us that particles moving
  in circles lose energy via radiation
• Therefore, electrons should end up spiraling into
  nucleus

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Orbital Atom

• 1913 1930
• Niels Bohr
• Electrons confined to energy levels (orbits)
• Electrons can change levels providing conserve
  energy

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Back to Rutherford

• Alpha particles into nitrogen gas, 1918
• Anomaly hydrogen nuclei coming off N
• N O hydrogen nuclei
• Discovery of proton, protos first
• Discovery of transmutation
• Predicts neutron in 1920

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Back to Rutherford

• It seems very likely that one electron can
  also bind two hydrogen nuclei and possibly also
  one hydrogen nucleus. In the one case, this
  entails the possible existence of an atom of mass
  nearly 2 carrying one charge, which is to be
  regarded as an isotope of hydrogen. In the other
  case, it involves the idea of the possible
  existence of an atom of mass 1 which has zero
  nuclear charge.

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Such an atomic structure seems by no means
impossibleThe existence of such atoms seems
almost necessary to explain the building up of
the nuclei of heavy elements for unless we
suppose the production of charged particles of
very high velocities it is difficult to see how
any positively charged particle can reach the
nucleus of a heavy atom against its intense
repulsive field. Andrade, 1964, pg. 169
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James Chadwick

• Rutherfords student
• Repeated the experiments of Irene and Frederic
  Joliot-Curie
• Stream of particles at beryllium which emitted
  rays with an intense capacity for penetrating
• Electrically neutral
• Named neutron, 1932

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The accelerator

• New invention
• Uses electric and magnetic fields to accelerate
  charged subatomic particles or nuclei to high
  energy
• Rutherford told Bohr
• when lightweight elements are bombarded with
  protons, they fragment into two ordinary alpha
  particles
• Bohr concluded
• For a brief instant, the light elements nuclei
  accepted the incoming proton. Then, an alpha
  particle (two protons and two neutrons) was
  emitted.

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Patrick Blackett Carl D. Anderson

• 1933
• Independently discovered positron
• Positively charged electron
• cosmic rays in a magnetic field produced both
  right and left curved tracks
• First anti-particle to be discovered

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Irene and Frederic Joliot-Curie

• 1934
• Artificial radioactivity
• Bombed aluminum with alpha particles
• Produces phosphorus that vanishes to silicon in
  half an hour
• Releases gamma rays and positron

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Enrico Fermi

• Repeated Joliot-Curie experiment with neutrons
  instead of alpha particles
• Increase chance of hitting a nucleus with more
  force
• Fired neutrons into uranium
• Uranium became very active and released an
  unknown element
• Discovery of trans-uranic elements

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Nuclear Reactions

• Bohr, again
• Nucleus is compound
• Nuclear reactions occur in two stages
• Entering particle collides with nuclear particle
• Nuclear particles collide with one another until
  all particles are in motion and original is one
  of all
• One particle near edge receives enough energy to
  escape from nucleus, releasing energy
• Predicts atomic bomb

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Bohr Predicts Atomic Bomb

• "If still more violent impacts could be
  conceived, say with particles of 1,000 million
  volts, then we must be prepared for the collision
  to lead to an explosion of the whole nucleus.
• Moore, 1985, pg. 213

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Louis-Victor de Broglie

• Particles have properties of waves
• Wave-particle duality of nature
• Applies to all waves and all particles
• The more massive the particles, the less obvious
  the wave properties. 
• Electrons, having very little mass, exhibit
  significant wave-like properties.

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Werner Heisenberg

• 1927
• Heisenbergs Uncertainty Principle
• Also known as Principle of Indeterminacy
• Cant know both position and velocity

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Back to Atomic Models

• Problems with Bohrs Orbital Atom
• Evidence against it was mounting
• only useful for predicting the behavior of atoms
  with a single electron
• Didnt explain wave-particle duality
• Disregarded Heisenberg Principle
• One dimension didnt translate well to reality

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Erwin Schrödinger

• 1926
• Schrödinger Wave Equation
• describes the  form of the probability waves that
  govern the motion of small particles and how
  these waves are altered by external influences

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Electron Cloud Model

• 1926 present
• Erwin Schrödinger
• Replaced circular orbits with new orbitals
• regions in space where electrons are most likely
  to be found
• Three dimensional
• Based on four quantum numbers
• Principal
• Orbital
• Magnetic
• Spin
• Based on probabilities

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Wolfgang Pauli

• Paulis Exclusion Principle
• No two electrons can have identical quantum
  numbers
• Instead of using all four quantum numbers to talk
  of electrons, scientists use shells and subshells
• Shell states with same principal number
• Subshell states within shell with same orbital
  number
• Can use refined orbital atom or electron cloud
  model

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Where Are We Today?

• We live in the electron cloud model paradigm
• We know it isnt perfect
• Few truly understand complexity of atom or
  quantum mechanics
• Bohr says
• "Those who are not shocked when they first come
  across quantum theory cannot possibly have
  understood it (Haselhurst, 2006, par. 3)."

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Where Are We Today?

• To date, scientists have discovered about two
  hundred sub-atomic particles consisting of matter
  and anti-matter
• Protons and neutrons are composed of three quarks
• Electrons are a type of lepton
• Quarks and leptons are currently believed to be
  the most fundamental particles

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Conclusions

• Complexity cant be an excuse
• What discovery comes next?
• What about the future of the atom?
• Star Trek type stuff???? Teleportation???

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ConclusionsWhat do you want me to do about it?

• Teach the next generation of nuclear physicists
  and nuclear chemists!
• It is imperative that today's students are taught
  the true evolution of the building block of life.

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Final Thought

• From the Greeks to Schrödinger, the question
  has been, What makes up the world? Ideas
  indeed need men to live and, if equipped with
  atomic education, a student sitting at a desk
  today can be the next to answer.
• Sarah L. Wood

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Works Cited
Andrade, Edward Neville da Costa. Rutherford
and the Nature of the Atom. Garden City
Doubleday/Anchor, 1964. Greenaway, Frank. John
Dalton and the Atom. Ithaca Cornell University
Press, 1966. Haselhurst, Geoffrey. "Quantum
Theory." 2006. The Philosophy Shop. 6 Dec.
2006 lthttp//www.spaceandmotion.com/quantum-theor
y-max-planck- quotes.htmgt Kuhn, Thomas S. The
Structure of Scientific Revolutions. 3rd ed.
Chicago University of Chicago Press,
1996. Moore, Ruth E. Niels Bohr, The Man, his
Science the World They Changed. Cambridge
MIT Press, 1985.
51
Images Courtesy of
Bowden, Mary Ellen. "Antoine-Laurent Lavoisier."
2005. Chemical Heritage Foundation. 6 Dec.
2006 lthttp//www.chemheritage.org/classroom/ chem
ach/forerunners/lavoisier.htmlgt Cornelio, Alicia
and Cornelio, Langlais. Atomic Models
Webquest. 2002. Manchester High School. 9
Dec. 2006 lthttp//mhsweb.ci. manchester.ct.us/Li
brary/webquests/atomicmodels.htmgt OConnor, J.
J. and Robertson, E. F. The MacTutor History of
Mathematics Biographies. Nov. 2002.
University of St. Andrews. 9 Dec. 2006
lthttp//www-history.mcs.st-andrews.ac.ukgt
Nobel Lectures, Physics 1901-1921. Amsterdam
Elsevier Publishing Company, 1967 AJ Software
Multimedia. The Mushroom Cloud. 2006.
National Science Foundation. 10 Dec. 2006
lthttp//www.atomicarchive.com/ Effects/effects9.s
htmlgt