The Atom

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Chapter 4

• The Atom Part 1

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In Search of Giants Part 1 Atoms and the
Periodic Table (202)
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What is the atom?

• Sulfur

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What is the atom?

• Lead

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• An atom is the smallest particle of an element
  that retains that elements properties.
• From the Greek a-tomos which means not
  divisible.
• The concept of the atom was first proposed in
  Greece over 5000 years ago.

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Democritus

• First proposed the concept that matter was not
  continuous but was instead made up of tiny
  particles he termed the atoms.

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Aristotle

• Believed in the four elements Earth, Air, Fire
  and Water.
• Proposed a fifth element ether.
• Because of Aristotles vast influence on later
  generations his ideas would persist until the
  Renaissance. (Philosophy vs. Science)

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Sir Francis Bacon Teacher Facts

• Member of British Parliament and Attorney General
  of England charged with corruption many think
  wrongly charged by his political enemies.
• Changed philosophy to science - deductive to
  inductive reasoning ( ideas and feelings to
  fact).
• Died of pneumonia experimenting with snow
  jumped out of his carriage in the mountains to
  experiment on it.

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Sir Francis Bacon (1561-1626)

• Father of Inductive Reasoning.
• In deductive reasoning, a conclusion is reached
  from general statements, but in inductive
  reasoning the conclusion is reached from specific
  examples.
• Knowledge is Power

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Robert Boyle Teacher Facts

• Wrote The Skeptical Chymist in 1661 appealed
  for experimentation to be the basis of the new
  science of chemistry.
• Influential in moving alchemy to chemistry.

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Robert Boyle (1627 1691)

• Robert Boyle is generally regarded as the first
  modern chemist

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Antoine Lavoisier (1743 1794)

• Father of Modern Chemistry.
• Law of Conservation of Mass.
• Abolished the Phlogiston Theory.
• Recognized and named oxygen and hydrogen.
• Introduced the metric system.
• Wrote the first extensive list of elements.

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Antoine Lavoisier (1743 1794)

• Declared a traitor during The Reign of Terror.
• It took them only an instant to cut off his
  head, but France may not produce another like it
  in a century (LaGrange).

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John Dalton (1766 1844)

• Proposed the first scientifically supported
  atomic theory.

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Daltons Model of the Atom

• Dalton's model was that the atoms were tiny,
  indivisible, indestructible particles and that
  each one had a certain mass, size, and chemical
  behavior that was determined by what kind of
  element they were.
• What made us change Daltons Atomic Model?

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Cathode Ray Tube

• Physicists in the 19th century found out that if
  they constructed a glass tube with wires inserted
  in both ends, and pumped out as much of the air
  as they could, an electric charge passed across
  the tube from the wires would create a
  fluorescent glow.

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Cathode Ray Tubes
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Scientific Debate

• William Crookes discovered that a tube coated in
  a fluorescing material at the positive end, would
  produce a focused dot when cathode rays hit it.
• With more experimentation, researchers found that
  the cathode rays moved with the properties of
  waves. However, other researchers argued that the
  focused nature of the cathode ray beam meant that
  they had to be made up of particles.

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Scientific Debate

• Physicists knew that the ray carried a negative
  charge but were not sure whether the charge could
  be separated from the ray. They debated whether
  the rays were waves or particles, as they seemed
  to exhibit some of the properties of both. In
  response, J. J. Thomson constructed some unique
  experiments to find a definitive and
  comprehensive answer about the nature of cathode
  rays.
• Wave Particle Duality is a concept that will be
  discussed at a later time.

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THOMSONS FIRST CATHODE RAY EXPERIMENT

• Thomson built a cathode ray tube with an
  electrometer, a device for catching and measuring
  electrical charge.

electrometer
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THOMSONS FIRST CATHODE RAY EXPERIMENT

• Thomson wanted to see if, by bending the rays
  with a magnet, he could separate the charge from
  the rays.
• He found that when the rays were bent toward the
  electrometer that it registered a large amount of
  negative charge and that when they were bent away
  the electrometer did not register much electric
  charge.
• This showed that the negative charge and the
  cathode rays must somehow be stuck together.

electrometer
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THOMSONS FIRST CATHODE RAY EXPERIMENT

• Thomsons first experiment therefore allowed him
  to speculate that the cathode rays were made of
  negatively charged particles.

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THOMSONS SECOND CATHODE RAY EXPERIMENT

• Thomson developed the second stage of the
  experiment, to prove that the rays carried a
  negative charge.
• To prove this hypothesis, he attempted to deflect
  the cathode rays with an electric field. Earlier
  experiments had failed to back this up, but
  Thomson thought that the vacuum in the tube was
  not good enough, and found ways to improve the
  quality of the vacuum.

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THOMSONS SECOND CATHODE RAY EXPERIMENT
anode
cathode

• Thomson's second experiment proving cathode rays
  had electric charge.
• Cathode rays have charge they are attracted
  toward the positive plate D and repelled by the
  negative plate E.

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THOMSONS SECOND CATHODE RAY EXPERIMENT

• Thomson concluded from his first two experiments
  "I can see no escape from the conclusion that
  cathode rays are charges of negative
  electricity carried by particles of matter."
• By deflecting the cathode rays with the electric
  charge, Thomsons second experiment proved, that
  the cathode rays were made up of negatively
  charged particles.

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THOMSONS SECOND CATHODE RAY EXPERIMENT

• This result was a major discovery in itself, but
  Thomson resolved to understand more about the
  nature of these particles.
• Thomson asked "What are these particles? Are
  they atoms, or molecules, or matter in a still
  finer state of subdivision?
• He then moved on to a third experiment.

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THOMSONS THIRD CATHODE RAY EXPERIMENT

• Thomson's third experiment sought to determine
  the basic properties of the particles.
• He could measure how much the rays were bent by a
  magnetic field, and how much energy they carried.
  
• From this data he could calculate the ratio of
  the mass of a particle to its electric charge
  (m/e).

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THOMSONS THIRD CATHODE RAY EXPERIMENT

• Thomson found out that the mass to charge ratio
  was so small that the particles had to be nearly
  2000 times smaller than a hydrogen atom.
• Do you understand the significance of this?

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THOMSONS THIRD CATHODE RAY EXPERIMENT

• Thomson speculated that the cathode rays were
  made of negatively charged particles that must
  come from within the atoms. Since they were much
  smaller than the atom itself.
• Therefore the atom was not the smallest particle
  of matter and could be broken apart.
• He had discovered the first subatomic particle.
• The Electron

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THOMSONS CATHODE RAY EXPERIMENTS

• Thomsons work with cathode rays shows how a
  series of experiments can gradually uncover
  truths.
• Many great scientific discoveries involve
  performing a series of interconnected
  experiments, gradually accumulating data and
  proving a hypothesis.

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THOMSONS CATHODE RAY EXPERIMENTS

• Thomson received the Nobel prize for physics in
  1906 for this work.

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J. J. Thomson (1856 1940)

• Credited for the discovery of the electron.
• Invented the mass spectrometer which led to his
  discovery of isotopes.

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Thomsons Model of the Atom

• Thomsons model of the atom stated that the atom
  is a tiny, spherical and neutral body, which is
  made up of negatively charged particles
  (electrons) in a positive mass which neutralized
  the charge of the negative electrons.
• This model is commonly referred to as the
  plum-pudding model of the atom.

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In Search of Giants Part 2 The Discovery of the
Electron (250)
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James Chadwick (1891 1974)

• Discovered the neutron.
• Paved the way for nuclear power and the nuclear
  bomb.

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Protons

• Somewhere between Thomson and Chadwick,
  physicists realized that there are positively
  charged particles in the atom which we call
  'protons'.
• The way this happened was a gradual process, and
  that is why it is hard to say exactly who
  discovered the proton, although Ernest
  Rutherford, is often given credit.

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Lord Ernest Rutherford Teacher Facts

• Died in hospital from complications of a hernia.
• British protocol required that he be operated on
  by a titled doctor since he was a peer. This
  delay cost him his life.

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Lord Ernest Rutherford (1871 1937)

• Discovered the nucleus of the atom.
• Pioneered the orbital theory (planetary model) of
  the atom.

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Rutherfords Gold Foil Experiment
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Rutherfords Gold Foil Experiment
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Rutherfords Gold Foil Experiment
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Rutherfords Atomic ModelThe Planetary Model
of the Atom

• The nucleus is very small, dense, and positively
  charged.
• Electrons surround the nucleus.
• Most of the atom is empty space

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In Search of Giants Part 3 The Discovery of the
Nucleus (320)