Covalent Network Solids

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 arraysheets 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 AcidsInert to Common BasesNegative Electron Affinity TransparentHardestBest Thermal ConductorLeast CompressibleStiffest

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Allotropes of Carbon - Graphite

Soft and Greasy FeelingSolid Lubricant Pencil “Lead”Conducts ElectricityReacts with Acids and Oxidizing Agents

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Allotropes of Carbon - Buckminsterfullerene

Sublimes between 800°CInsoluble in waterSoluble in tolueneStable in air Requires temps > 1000°C to decomposeHigh electronegativity Reacts with alkali metalsBehavior 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 elementsused 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 Csimilar size to Caverage electronegativity of B & N about the same as Celectrical 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 = Ca2B6O115H2O

• 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 ringmelts at 450C

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 Oand minor amounts of other elementsfound in rocks, soils, and clayssilicates 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 hybridizationtetrahedral shapeSi-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 1500C and cooled quickly, get amorphous silica which we call glass

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Single Chain Silicates• if the SiO4

4- 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 chainsdiopside = CaMg(SiO3)2 where Ca

and Mg occupy lattice points between the chains QuickTime™ and a

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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 mineralsasbestostremolite 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