Structure of solids continued

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Chapter 11

• Structure of solids continued

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Structure and Bonding in Metals

• Metals have
• High thermal and electrical conductivity
• Are malleable
• Are Ductile
• The reason for this is they are like small
  spheres packed together and bonded equally with
  other metal atoms in all directions.

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Body-centered Face-centered Crystal Lattice
                                    
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Closest Packing

• The structural model has uniform spheres as atoms
  packed in a manner that most efficiently uses the
  available space.
• The top layer does not lie directly on the
  spheres below but in the spaces available.

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Hexagonal close packing

• When the atoms in the third layer lay over the
  atoms in the first layer.
• The unit cell here is body centered.

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Other examples in nature of Hexagonal Close
Packing
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Cubic Close packing

• When the first and the fourth layer line up with
  one another.
• The unit cell shown is face centered cubic.

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Bonding Model for Metals

• Metals qualities are best explained by the
  electron sea model.
• This envisions a regular array of organized
  cations surrounded by delocalized sea of
  electrons.
• This allows the movement of electrical current,
  and the metal ions can be easily moved around as
  a metal is hammered into a shape.

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Metal Strength

• Sodium, potassium and lithium are soft metals
  that may be cut with a spoon! They have only one
  valence electron each.
• Chromium and iron are much harder metals each
  with 6 and 8 valence electrons respectively.
• What about mercury?

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Discussion

• Mercury hangs on to its valence 6s electrons very
  tightly. Mercury-mercury bonding is very weak
  because its valence electrons are not shared
  readily. (In fact mercury is the only metal that
  doesn't form diatomic molecules in the gas
  phase).
• Hg 200.59 Kr 4d10 4f14 5s2 5p6 5d10 6s2

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Other notes

• Metal alloys are a substance that contains a
  mixture of elements and has metallic properties.
• There are two types of alloys
• Substitutional alloy
• Interstitial alloy

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Substitutional Interstitial
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Bonding in Molecular Solids

• Molecular solids are held together by
  intermolecular forces.
• London forces, Dipole-dipole and hydrogen
  bonding.
• The properties of the molecular solids depends
  not only on the strength of these forces but also
  on the ability of the molecules to closely pack.
• Examples Ar, CO2, and H2O

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Network Solids

• Many atomic solids form strong directional
  covalent bonds. This allows the formation of
  giant molecules.
• Silicon and Carbon form some of the most
  important network solids.
• Diamond and graphite are both made of carbon. Yet
  diamond is a poor conductor and graphite can
  conduct electricity.

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Why?

• Diamond is carbon bound in a tetrahedral shape to
  other carbons (sp3). This localizes the
  electrons and prevents conduction.
• Graphite is layers of 6 carbon rings with some
  delocalized electrons between the sheets of
  rings. Aka. sp2 hybridization with pi-bonds.

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This is why!
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Silica

• Silica (SiO2) crystal when heated to 1600 C and
  cooled rapidly an amorphous solid called glass is
  formed.

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Ionic Solids

• These are stable high melting substances held
  together by strong static forces between
  oppositely charged ions.
• Most are binary solids and can be modeled by
  closest packing spheres.
• The smaller cations fit in the holes created by
  closely packing the anions.
• The packing is done to maximize the oppositely
  charged particles and minimize the repulsions by
  ions with the same charge.

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Shapes

• There are three types of holes in closest packed
  structures.
• Trigonal holes formed by three sphere in the same
  layer
• Tetrahedral holes formed when a sphere sits in
  the dimple of three spheres in an adjacent layer.
• Octahedral holes are formed by two sets of three
  spheres of the closest packed structure.
• The relative size of the wholes is
  Trigonallttetrahedralltoctahedral

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