Covalent Network Solids

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Covalent Network Solids
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Carbon

• exhibits the most versatile bonding of all the
  elements
• diamond structure consists of tetrahedral sp3
  carbons in a 3-dimensional array
• graphite structures consist of trigonal planar
  sp2 carbons in a 2-dimensional array
• sheets attracted by weak dispersion forces
• fullerenes consist of 5 and 6 member carbon rings
  fused into icosahedral spheres of at least 60 C

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Allotropes of Carbon - Diamond
Inert to Common Acids Inert to Common
Bases Negative Electron Affinity
Transparent Hardest Best Thermal Conductor Least
Compressible Stiffest
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Allotropes of Carbon - Graphite
Soft and Greasy Feeling Solid Lubricant Pencil
Lead Conducts Electricity Reacts with Acids and
Oxidizing Agents
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Allotropes of Carbon - Buckminsterfullerene
Sublimes between 800C Insoluble in water Soluble
in toluene Stable in air Requires temps gt 1000C
to decompose High electronegativity Reacts with
alkali metals Behavior more aliphatic than
aromatic
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Nanotubes

• long hollow tubes constructed of fused C6 rings
• electrical conductors
• can incorporate metals and other small molecules
  and elements
• used to stabilize unstable molecules
• single-walled nanotubes (SWNT) have one layer of
  fused rings
• multi-walled nanotubes (MWNT) have concentric
  layers of fused rings

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Nanotubes
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Nanotubes

• carbon nanotube sp2 C in fused hexagonal rings
• electrical conductors
• boron-nitride nanotubes rings of alternating B
  and N atoms
• isoelectronic with C
• similar size to C
• average electronegativity of B N about the same
  as C
• electrical insulators

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Insulated Nanowire
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Boron

• metalloid
• less than 0.001 in Earths crust, but found
  concentrated in certain areas
• almost always found in compounds with O
• borax Na2B4O5(OH)4?8H2O
• kernite Na2B4O5(OH)4?3H2O
• colemanite Ca2B6O11?5H2O
• used in glass manufacturing borosilicate glass
  Pyrex
• used in control rods of nuclear reactors

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Boron-Oxygen Compounds

• form 2D structures with trigonal BO3 units
• in B2O3, six units are linked in a flat hexagonal
  B6O6 ring
• melts at 450?C
• melt dissolves many metal oxides and silicon
  oxides to form glasses of different compositions

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Silicates

• the most abundant elements of the Earths crust
  are O and Si
• silicates are covalent atomic solids of Si and O
• and minor amounts of other elements
• found in rocks, soils, and clays
• silicates have variable structures leading to
  the variety of properties found in rocks, clays,
  and soils

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Bonding in Silicates

• each Si forms a single covalent bond to 4 O
• sp3 hybridization
• tetrahedral shape
• Si-O bond length is too long to form SiO
• to complete its octet, each O forms a single
  covalent bond to another Si
• the result is a covalent network solid

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Quartz

• a 3-dimensional covalent network of SiO4
  tetrahedrons
• generally called silica
• formula unit is SiO2
• when heated above 1500?C and cooled quickly, get
  amorphous silica which we call glass

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Single Chain Silicates

• if the SiO44- units link as long chains with
  shared O, the structure is called a pyroxene
• formula unit SiO32-
• chains held together by ionic bonding to metal
  cations between the chains
• diopside CaMg(SiO3)2 where Ca and Mg occupy
  lattice points between the chains

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Double Chain Silicates

• some silicates have 2 chains bonded together at ½
  the tetrahedra these are called amphiboles
• often results in fibrous minerals
• asbestos
• tremolite asbestos Ca2(OH)2Mg5(Si4O11)2

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Sheet Silicates

• when 3 O of each tetrahedron are shared, the
  result is a sheet structure called a
  phyllosilicate
• formula unit Si2O52-
• sheets are ionically bonded to metal cations that
  lie between the sheets
• talc and mica

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Mica a Phyllosilicate