Covalent Network Solids

Covalent Networks A continuous array of covalently bonded atoms 3-D Covalent Crystals Diamond - C Each Carbon forms 4 sp3 bonds to another C in a tetrahedral shape Structure is continuous in 3 dimensions Very hard due to small atoms (strong force of attraction) and repeated bonding structure Localized valence e- in pure covalent bonds - NO CONDUCTIVITY!

Silicon - Si Same configuration as Diamond Large atoms result in weakened force of attraction to valence e-s Not as hard as Diamond

Silicon Carbide – SiC Same configuration as Diamond Alternating Si and C atoms Between Diamond and a Si network solid in hardness Ceramic used for car brakes, sandpaper and grinding wheels.

Quartz- SiO2 Silicon surrounded by 4 oxygen atoms in a sp3 tetrahedral shape and each O is bonded to 2 Si. Impurities can give coloured quartz When heated at very high temperatures, glass is created.

Application – Doping of Silicon Insertion of a 5 e- atom into some of the Si sites provides an extra, delocalized e- for an n-type semi-conductor (negative charge carrying) Insertion of a 3 electron atom into some of the Si sites provides an e- vacancy or “hole” for a p-type semi-conductor (positive charge carrying) Junctions between n-doped and p-doped materials can be used to control e- flow, and are the basis of modern electronics

2-D Covalent Crystals Graphite – C Each Carbon bonds to 3 other Cs in a trigonal planar shape – they form layers The 4th e- in each C is delocalized in a perpendicular atomic p orbital which allows for conductivity.

Weak attractions between the layers of C allow for slippage and for a layer of C to be removed easily. Used as a lubricant, in pencils, in nuclear reactors. No conduction across the sheets only between the sheets.

Nanotubes – C6 long hollow tubes constructed of fused C6 rings electrical conductors also used to stabilize unstable molecules

Mica & Clay – Si2O52− Similar to Quartz but 1 of the 4 Oxygen atoms bonded to Si does not bridge to another Si but retains a negative charge which allows for conductions between the sheets. Material will easily flake into sheets. Used in electrical components and in applications where electrical conductivity and durability in high temperatures is required.

1-D Covalent Crystals Asbestos – SiO32− Only 1 Oxygen bonded to Si bridges to another Si Structure is fibrous and can be pulled apart into strings easily It was used extensively due to its physical properties: sound absorption, tensile strength, resistance to fire, heat, electricity, and it could be woven into fabric and mats. It was found to have extreme health risks when in powder or dust forms. Lung cancer and asbestosis (crystallization of lung cilia) caused its use to be discontinued in the 1980’s.