Chapter 5: Atomic Structure and the Periodic Table

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Chapter 5 Atomic Structure and the Periodic Table
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Early Models of Atoms

• Democritus (460-400B.C.) first suggested the
  existence of these particles, which he called
  atoms for the Greek word for uncuttable. They
  lacked experimental support due to the lack of
  scientific testing at the time.
• John Dalton (1766-1844) performed experiments to
  study the ratios in which elements combine in
  chemical reactions. Formulate hypotheses and
  theories to explain his observations, which
  became Daltons Atomic Theory.
• All elements are composed of tiny indivisible
  particles called atoms.
• Atoms of the same element are identical. The
  atoms of any one element are different from those
  of any other element.
• Atoms of different elements can physically mix
  together or combine in simple, whole number
  ratios to form compounds.
• Chemical reactions occur when atoms are
  separated, joined or rearranged. Atoms of one
  element, however, are never changed into atoms of
  another element as a result of a chemical
  reaction.

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Size of an Atom

• Imagine grinding a copper coin (penny) into fine
  dust. Each speck in the small pile of shiny red
  dust would still have the properties of copper.
  If by some means you could still make the dust
  particles smaller you would eventually come upon
  a particle known as an atom.
• An atom is the smallest particle of an element
  that retains the properties of that element.
• A pure copper penny contains about 2.4 X 1022
  atoms, compared to the Earths population of 6 X
  106 people.
• If you lined 100,000,000 copper atoms up side by
  side they would produce a line 1 cm long.

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Atomic Structure

• Atoms are now known to be divisible as they can
  be broken down to even smaller particles by atom
  smashers.
• J.J. Thomson (1856-1940) discovered electrons
  using cathode ray tubes.
• Robert Millikan (1868-1953) carried out
  experiments to determine the charge of an
  electron (-). He also determined the ratio of the
  charge to the mass of an electron.
• In 1886, E. Goldstein observed a cathode ray
  tube and found rays traveling in the opposite
  direction to that of the cathode rays. He called
  these rays canal rays and concluded that they
  must be positive particles, which are now called
  protons.
• In 1932, James Chadwick confirmed the existence
  of yet another subatomic particle the neutron.
  Neutrons are subatomic particles with no charge
  but with a mass nearly equal to that of a proton.
  See simulation

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• After discovering these subatomic particles,
  scientists wondered how they were put together.
• JJ Thompson thought since the electrons
  contributed such a small fraction of the atoms
  mass, they were probably an equal fraction of it
  size so it was like Plum Pudding.
• In 1911, Ernest Rutherford and his coworkers
  performed the Gold Foil Experiment to further
  study the phenomenon.
• Concluded that most of the mass of each atom and
  all of its positive charge reside in a very
  small, extremely dense region which is called the
  nucleus. The rest of the atom is mostly empty
  space.

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Modern View of Atomic Structure

• Since the time of Rutherford, physicists have
  learned much about the nucleus. Although many
  other parts have been discovered, chemists tend
  to only work with three main particles since they
  determine chemical behavior Electron, Neutron
  and Proton
• Electron has a charge of -1.602 X 10-19 C and a
  proton has a charge of 1.602 X 10-19 C so this
  quantity of Coulombs is known as one electronic
  charge and atomic and subatomic particles usually
  have a charge that is multiples of this. Neutrons
  have no charge and are electrically neutral.
• Atoms have extremely small masses so instead of
  using the real numbers, atomic mass units (amus)
  are used. Protons and neutrons are very similar
  in mass but it would take 1836 electrons to equal
  1 proton so most of an atoms mass is in the
  nucleus.
• Atoms are also extremely small with diameters
  between 1 X 10-10 and 5 X 10-10 so they are
  usually expressed with angstroms, which is 10-10.
  

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Atomic Number

• The number of protons in the nucleus of an atom
  of that element
• For an atom with no charge, this is also the
  number of electrons since the positive charge of
  the protons cancels the negative charge of the
  electrons.
• Practice problems 7-8 pg 115

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Mass Number

• Most of the mass of an atom is found in the
  nucleus so the total number of protons and
  neutrons equals the mass number.
• If you know the atomic umber and mass number you
  can determine the composition of that atom.
• The composition can be represented by the
  shorthand notation using the element symbol,
  atomic number and mass number.
• For gold, Au is the symbol for the element and
  the atomic number is subscript and mass number is
  superscript on the left side.

Au
197
79

• Practice problems 9-11 pg 116

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Isotopes

• Atoms that have the same number of protons but
  different number of neutrons.
• Affects the shorthand notation of the element.
• Practice problems 12-13 on pg 117

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Atomic Mass

• Today we can determine the masses of individual
  atoms with a relative high degree of accuracy but
  since they are so small atomic mass units are
  used with hydrogen being 1 amu.
• The average atomic mass for an element due to the
  different isotopes, the mass of those isotopes
  and the natural percent abundance.
• Add up the different atomic mass of each atom and
  then divide by the number of atoms.
• Or, multiply mass by and then determine average
  mass.
• Practice problems 14-15 pg 120 and 16-17 pg 121.

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Mass Spectrometer

• The most direct and accurate means for
  determining atomic and molecular weights. See pg
  48

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Periodic Table

• The arrangement of elements in order of
  increasing atomic number, with elements having
  similar properties placed in vertical column.
• Atomic number, symbol, name, atomic weight are
  found in each square for each element. Some
  tables have additional information as well.
  Example
• Can be arranged according to metals, non-metals
  and metalloids, solid liquid and gases, and by
  family. Example