Bonding in Solids

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Bonding in Solids

• There are four types of solid
• Molecular (formed from molecules) - usually soft
  with low melting points and poor conductivity.
• Covalent network (formed from atoms) - very hard
  with very high melting points and poor
  conductivity.
• Ions (formed form ions) - hard, brittle, high
  melting points and poor conductivity.
• Metallic (formed from metal atoms) - soft or
  hard, high melting points, good conductivity,
  malleable and ductile.

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Figure 10.12Examples of Three Types of
Crystalline Solids
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Bonding in Solids
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Bonding in Solids

• Molecular Solids
• Intermolecular forces dipole-dipole, London
  dispersion and H-bonds.
• Weak intermolecular forces give rise to low
  melting points.
• Room temperature gases and liquids usually form
  molecular solids and low temperature.
• Efficient packing of molecules is important
  (since they are not regular spheres).

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Bonding in Solids

• Covalent Network Solids
• Intermolecular forces dipole-dipole, London
  dispersion and H-bonds.
• Atoms held together in large networks.
• Examples diamond, graphite, quartz (SiO2),
  silicon carbide (SiC), and boron nitride (BN).
• In diamond
• each C atom has a coordination number of 4
• each C atom is tetrahedral
• there is a three-dimensional array of atoms.
• Diamond is hard, and has a high melting point
  (3550 ?C).

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Bonding in Solids

• Covalent Network Solids

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Diamond
- Hardest Structure Known - Tetrahedral atomic
arrangement
What hybrid state do you think the carbon has?
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Bonding in Solids

• Covalent Network Solids
• In graphite
• each C atom is arranged in a planar hexagonal
  ring
• layers of interconnected rings are placed on top
  of each other
• the distance between C atoms is close to benzene
  (1.42 Å vs. 1.395 Å in benzene)
• the distance between layers is large (3.41 Å)
• electrons move in delocalized orbitals (good
  conductor).

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Graphite
- Form planar sheets - actually more
stable than diamond
What hybrid state do you think the carbon has?
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Bonding in Solids

• Ionic Solids
• Ions (spherical) held together by electrostatic
  forces of attraction
• The higher the charge (Q) and smaller the
  distance (d) between ions, the stronger the ionic
  bond.
• There are some simple classifications for ionic
  lattice types

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Bonding in Solids
Ionic Solids
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Bonding in Solids

• Ionic Solids
• NaCl Structure
• Each ion has a coordination number of 6.
• Face-centered cubic lattice.
• Cation to anion ratio is 11.
• Examples LiF, KCl, AgCl and CaO.
• CsCl Structure
• Cs has a coordination number of 8.
• Different from the NaCl structure (Cs is larger
  than Na).
• Cation to anion ratio is 11.

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Bonding in Solids
Ionic Solids
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Bonding in Solids

• Ionic Solids
• Zinc Blende Structure
• Typical example ZnS.
• S2- ions adopt a fcc arrangement.
• Zn2 ions have a coordination number of 4.
• The S2- ions are placed in a tetrahedron around
  the Zn2 ions.
• Example CuCl.
• Fluorite Structure
• Typical example CaF2.
• Ca2 ions in a fcc arrangement.
• There are twice as many F- per Ca2 ions in each
  unit cell.
• Examples BaCl2, PbF2.

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Bonding in Solids
Ionic Solids
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Bonding in Solids

• Metallic Solids
• Metallic solids have metal atoms in hcp, fcc or
  bcc arrangements.
• Coordination number for each atom is either 8 or
  12.
• Problem the bonding is too strong for London
  dispersion and there are not enough electrons for
  covalent bonds.
• Resolution the metal nuclei float in a sea of
  electrons.
• Metals conduct because the electrons are
  delocalized and are mobile.

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Bonding in Solids
Metallic Solids
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The Molecular Orbital Energy Levels Produced
When Various Numbers of Atomic Orbitals Interact
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The Band Model for Magnesium
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Partial Representation of the Molecular Orbital
Energies in A) Diamond and B) a Typical Metal
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Figure 16.33 Empty/filled MOs
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Silicon Crystal Doped with (a) Arsenic and (b)
Boron
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Figure 16.34 p-n junction