covalent network solids
DESCRIPTION
Covalent Network Solids. Carbon. exhibits the most versatile bonding of all the elements diamond structure consists of tetrahedral sp 3 carbons in a 3-dimensional array graphite structures consist of trigonal planar sp 2 carbons in a 2-dimensional array - PowerPoint PPT PresentationTRANSCRIPT
Covalent Network
Solids
Tro, Chemistry: A Molecular Approach 2
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
Tro, Chemistry: A Molecular Approach 3
Allotropes of Carbon - Diamond
Inert to Common AcidsInert to Common BasesNegative Electron Affinity TransparentHardestBest Thermal ConductorLeast CompressibleStiffest
Tro, Chemistry: A Molecular Approach 4
Allotropes of Carbon - Graphite
Soft and Greasy FeelingSolid Lubricant Pencil “Lead”Conducts ElectricityReacts with Acids and Oxidizing Agents
Tro, Chemistry: A Molecular Approach 5
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
Tro, Chemistry: A Molecular Approach 6
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
Tro, Chemistry: A Molecular Approach 7
Nanotubes
Tro, Chemistry: A Molecular Approach 8
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
Tro, Chemistry: A Molecular Approach 9
Insulated Nanowire
Tro, Chemistry: A Molecular Approach 10
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
Tro, Chemistry: A Molecular Approach 11
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
Tro, Chemistry: A Molecular Approach 12
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
Tro, Chemistry: A Molecular Approach 13
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
Tro, Chemistry: A Molecular Approach 14
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
Tro, Chemistry: A Molecular Approach 15
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
decompressorare needed to see this picture.
QuickTime™ and a decompressor
are needed to see this picture.
Tro, Chemistry: A Molecular Approach 16
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
Tro, Chemistry: A Molecular Approach 17
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
Tro, Chemistry: A Molecular Approach 18
Mica: a Phyllosilicate