Covalent Network Solids

Tro, Chemistry: A Molecular Approach2 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 sheets 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 Approach3 Allotropes of Carbon - Diamond Inert to Common Acids Inert to Common Bases Negative Electron Affinity Transparent Hardest Best Thermal Conductor Least Compressible Stiffest

Tro, Chemistry: A Molecular Approach4 Allotropes of Carbon - Graphite Soft and Greasy Feeling Solid Lubricant Pencil “Lead” Conducts Electricity Reacts with Acids and Oxidizing Agents

Tro, Chemistry: A Molecular Approach5 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

Tro, Chemistry: A Molecular Approach6 Nanotubes long hollow tubes constructed of fused C 6 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

Tro, Chemistry: A Molecular Approach7 Nanotubes

Tro, Chemistry: A Molecular Approach8 Nanotubes carbon nanotube = sp 2 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

Tro, Chemistry: A Molecular Approach9 Insulated Nanowire

Tro, Chemistry: A Molecular Approach10 Boron metalloid less than 0.001% in Earth’s crust, but found concentrated in certain areas almost always found in compounds with O  borax = Na 2 [B 4 O 5 (OH) 4 ]  8H 2 O  kernite = Na 2 [B 4 O 5 (OH) 4 ]  3H 2 O  colemanite = Ca 2 B 6 O 11  5H 2 O used in glass manufacturing – borosilicate glass = Pyrex used in control rods of nuclear reactors

Tro, Chemistry: A Molecular Approach11 Boron-Oxygen Compounds form 2D structures with trigonal BO 3 units in B 2 O 3, six units are linked in a flat hexagonal B 6 O 6 ring melts at 450  C  melt dissolves many metal oxides and silicon oxides to form glasses of different compositions

Tro, Chemistry: A Molecular Approach12 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

Tro, Chemistry: A Molecular Approach13 Bonding in Silicates each Si forms a single covalent bond to 4 O sp 3 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

Tro, Chemistry: A Molecular Approach14 Quartz a 3-dimensional covalent network of SiO 4 tetrahedrons generally called silica formula unit is SiO 2 when heated above 1500  C and cooled quickly, get amorphous silica which we call glass

Tro, Chemistry: A Molecular Approach15 Single Chain Silicates if the SiO 4 4- units link as long chains with shared O, the structure is called a pyroxene formula unit SiO 3 2- chains held together by ionic bonding to metal cations between the chains diopside = CaMg(SiO 3 ) 2 where Ca and Mg occupy lattice points between the chains

Tro, Chemistry: A Molecular Approach16 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 = Ca 2 (OH) 2 Mg 5 (Si 4 O 11 ) 2

Tro, Chemistry: A Molecular Approach17 Sheet Silicates when 3 O of each tetrahedron are shared, the result is a sheet structure called a phyllosilicate formula unit = Si 2 O 5 2− sheets are ionically bonded to metal cations that lie between the sheets talc and mica

Tro, Chemistry: A Molecular Approach18 Mica: a Phyllosilicate