Atomic theory and the periodic table

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Atomic theory and the periodic table
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Current model of the atom a 3-D view of the atom
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How did we get to this point?
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Democritus or Leucippus

• Democritus was a student of Leucippus and
  co-originator of the belief that all matter is
  made up of various imperishable, indivisible
  elements which he called atoma (sg. atomon) or
  "indivisible units", from which we get the
  English word atom.

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Aristotle
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Did he agree with the theory of atoma?

• NO He believed in the idea that matter
  consisted of 4 basic elements Earth, Air, Water,
  and Fire. This was stated as part of his
  Doctrine of four elements.

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Who did the people believe?

• Because of his political influence and the
  influence of the church, many believed in the
  ideas of Aristotle for the next 2000 years. They
  followed what we now call Aristotelian
  philosophy.

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John DaltonSeptember 6, 1766 July 27, 1844)
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What did Dalton do?

• Dalton loved meteorology. Much of what he
  understood about matter was a result of his
  studies in this field.
• He arrived at Daltons Law of Partial pressures
  that stated that if you have a mixture of gases
  in a container, the total pressure was the sum of
  the individual partial pressures of the gas.
• He was able to calculate atomic weights (that
  were later used to develop the first periodic
  table).

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What was Daltons ,most significant contribution?

• John Dalton, in 1803, developed the first Modern
  Atomic Theory. This law had several components
• All matter is made up of atoms that are in
  continual motion and not capable of being
  sub-divided.
• The reason a form of matter is pure is pure is
  due to the fact that all atoms that make up that
  substance are identical.
• Atoms of different elements are different from
  each other.
• Compounds have constant composition because they
  have a fixed ratio of atoms.
• Chemical reactions involve a rearrangement of
  atoms.

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John Joseph Thomson(1856-1940)
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How did Thomson help to contribute to our
understanding of the atom?

• Using a high vacuum cathode-tube (see next slide)
  and electrical current, Thomson realized that
  there must be bodies must smaller than atoms
  that had a negative charge. He called these
  little bodies corpuscles that were later
  renamed electrons.
• This was significant in that it proves that there
  are lighter forms of matter than atoms.

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What is a cathode-ray tube
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Robert Millikan(1868 1953)
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What were Millikans contributions to our
understanding of the atom?

• In 1910, using the famous oil-drop experiment,
  he determined that all electrons in atoms had a
  fixed charge. Using some mathematics, he also
  calculated the charge of the electron.
• Between 1912 and 1915, he made the first direct
  photoelectric determination of Planck's constant,
  h.

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The Oil-Drop experiment
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Ernest Rutherford(1871-1937)
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What were his contributions to our understanding
of the atom?

• He was a research student working under Thomson
  while at the Cavendish Laboratory.
• In 1911, using alpha particles (positively
  charged form of radiation), he concluded that the
  atom is not a positive particle with negatively
  charged electrons just randomly scattered about
  as Thomson suggested in his Plum Pudding model
  of the atom.
• In 1916, he states publicly that he hoped
  mankind should not discover how to extract the
  energy from the nucleus until man was living at
  peace with his neighbour

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Neils Bohr(1885-1962)
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What were Bohrs contributions to our
understanding of the atom?

• He was a research student working under
  Rutherford.
• In 1913 Bohr published a theory about the
  structure of the atom based on an earlier theory
  of Rutherford's. Rutherford had shown that the
  atom consisted of a positively charged nucleus,
  with negatively charged electrons in orbit around
  it. Bohr expanded upon this theory by proposing
  that electrons travel only in certain
  successively larger orbits. He suggested that the
  outer orbits could hold more electrons than the
  inner ones, and that these outer orbits determine
  the atom's chemical properties.

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James Chadwick(1891 1974)
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What was Chadwicks work in helping us understand
about the structure of the atom?

• James Chadwick worked with Rutherford at
  Cambridge. Rutherford suggested the existence of
  neutrons, however, Chadwick was the first to
  isolate the neutron in 1932. He noticed the
  emission of a form of radiation from the nucleus
  of an atom that was NOT deflected by the presence
  of a magnetic field (like the positively charged
  alpha particle). This particle was called the
  neutron.

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Daltons model of the atom (similar to the early
Greeks)
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J.J. Thomsons model of the atom
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Rutherfords model of the atom
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Revised Rutherford model
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Bohrs Model of the atom
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Current model of the atom
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The periodic table

• A small 1-day unit dedicated to understanding how
  the periodic table has developed into what we now
  know it to look like.

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How did the table get to look like this?
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Since Dalton first determined a way to determine
the average weight of an element, scientists have
tried for years to link their weight to their
observed properties. There were three
breakthrough attempts prior to the a table that
starts to resemble the current. These three will
be discussed in class.

• History of Elements Classification Schemes
• Concept of Triads
• Law of Octaves
• Mendeleevs periodic table the first published
  table
• The current table

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Triads

• 1817
• According to Van Spronsen, the first observation
  of a relationship between the atomic weights
  (equivalent weight) of elements was made by
  Johann Wolfgang Dobereiner, of the University of
  Jena. He found three-member groups of analogous
  element (triads), for which the equivalent weight
  of the middle element was the arithmetic mean
  (average) of the weights of the other members of
  the triad. He concluded that this relationship
  must reflect some general principle--which he
  could not identify.

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Further study on Triads

• 1850
• Max von Pettenkofer, University of Munich,
  revives Prout's hypothesis of primary matter.
  Prout believed that all elements were whole
  number multiples of hydrogen. Hence, elements
  would not be indivisible, but have a smaller unit
  of structure. Pettenkofer expanded Dobereiner's
  triads. He noted that similar elements formed an
  arithmetic series and the difference between
  elements in each series was 8 or a multiple of 8.

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Law of Octaves

• 1865
• John Newlands constructs a system of
  classification based on octaves. He arranged
  elements in sets of eight in order of increasing
  average weights. His work was ridiculed as
  having no more basis than if he had chosen an
  alphabetical listing as a classification.

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Mendeleevs Table A breakthrough?!?!

• 1869
• Dmitri Mendeleev uses the advantage of recent
  developments--precise atomic weights (Cannizzaro)
  and the large sample of known elements (67) to
  create a system of classification based on atomic
  weight. Not only did he use relationships between
  elements with similar properties, but he also
  discovered relationships between elements with
  dissimilar properties. Whether or not Mendeleev
  used cards to sort his elements (his affinity for
  the card game patience is documented), he wrote a
  table which recognized bidirectional
  relationships, both horizontally and vertically.
• His predictions of the physical and chemical
  properties of undiscovered elements provided the
  experimental verification that made his theory
  accepted in just a few years. The discovery of
  gallium, germanium, and scandium brought
  acceptance where a purely theoretical base for
  the table could not.

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Below is what Mendeleevs Table might have looked
like.
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Why was Mendeleevs table considered a
breakthrough?

• At the time that Mendeleev developed his periodic
  table since the experimentally determined atomic
  masses were not always accurate, he reordered
  elements despite their accepted masses. For
  example, he changed the weight of beryllium from
  14 to 9. This placed beryllium into Group 2 above
  magnesium whose properties it more closely
  resembled than where it had been located above
  nitrogen. In all Mendeleev found that 17 elements
  had to be moved to new positions from those
  indicated strictly by atomic weight for their
  properties to correlate with other elements.

• Mendeleev left gaps in his table. From the gaps
  present in his table, Mendeleev predicted the
  existence and properties of unknown elements
  which he called eka-aluminum, eka-boron, and
  eka-silicon. The elements gallium, scandium and
  germanium were found later to fit his predictions
  quite well. In all Mendeleev predicted the
  existence of 10 new elements, of which seven were
  eventually discovered -- the other three, atomic
  weights 45, 146 and 175 do not exist.

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What was next???

• In 1895 Lord Rayleigh reported the discovery of a
  new gaseous element named argon which proved to
  be chemically inert. This element did not fit any
  of the known periodic groups.

• In 1898, William Ramsey suggested that argon be
  placed into the periodic table between chlorine
  and potassium in a family with helium, despite
  the fact that argon's atomic weight was greater
  than that of potassium. This group was termed the
  "zero" group due to the zero valency of the
  elements. Ramsey accurately predicted the future
  discovery and properties neon.

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Finally? (not yet)

• In 1913, Henry Moseley published the results of
  his measurements of the wavelengths of the x-ray
  spectral lines of a number of elements which
  showed that the ordering of the wavelengths of
  the x-ray emissions of the elements coincided
  with the ordering of the elements by atomic
  number. With the discovery of isotopes of the
  elements, it became apparent that atomic weight
  was not the significant player in the periodic
  law as Mendeleev, and others had proposed, but
  rather, the properties of the elements varied
  periodically with atomic number.

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Finally??? yes!!

• The last major changes to the periodic table
  resulted from Glenn Seaborg's work in the middle
  of the 20th Century. Starting with his discovery
  of plutonium in 1940, he discovered all the
  transuranic elements (elements from 94 to 102).
  He reconfigured the periodic table by placing the
  actinide series below the lanthanide series. In
  1951, Seaborg was awarded the Nobel Prize in
  chemistry for his work. Element 106 has been
  named seaborgium (Sg) in his honor.