covalent network - SCH4U1-CCVI

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covalent network - SCH4U1-CCVI
Covalent
Network
Solids
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 800°C
Insoluble in water
Soluble in toluene
Stable in air
Requires temps > 1000°C 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 Earth’s crust, but
found concentrated in certain areas
 almost always found in compounds with O
 borax = Na2[B4O5(OH)4]8H2O
 kernite = Na2[B4O5(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 Earth’s 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 Si=O
• 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 SiO32chains 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
QuickTime™ and a
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are needed to see this picture.
QuickTime™ and a
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are needed to see this picture.
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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
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