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# 2.2.7 Covalent Network Structures
1\. Molecular structure 2. Covalent networks ( giant covalent )
* Both are joined by covalent bonds
1. **Molecular** substances
* Consists of separate groups of covalent bonded atoms called **molecules**
2. **Covalent Networks ( Giant Covalent )**
* Contain covalently bonded atoms in a contiuous three-dimesional lattice.
* Silicon
* Silicon Dioxide
* And **allotropes of carbon**
**Allotropes of Carbon**
* Some elements have different structural forms called allotropes like Carbon
* Diamond
* Graphite
* Graphene
* Group of substances called fullerenes
* Despite all being made from carbon atoms, they have different **chemical** and **physical** properties due to their differing structural arrangemnets.
a- diamond b - graphite c - graphene d- nanotubes e- buckminsterfullerene (C_60)
1. **Diamond**
* Each carbon bonded to four others in tetrahedral arrangement
* Poor electrical conductor due to no mobile charged particles ( electrons are localized )
* Excellent Thermal Conductor
* Vibrational energy carriers called **phonons** travel welll through the highly egular lattice and strong covalent bonds
2. **Graphite**
* Made of sheets of carbon atoms
* Each carbon bonded to three others in a hexagonal arrangement where the geoemtry around each carbon atom is trigonal planar
* Are bonded such that one eelctron per carbon atom is delocalized
* Which are free to move up and down each sheet and $$\therefore$$a good electrical conductor
* The sheet itself is strong, but the attraction between the sheets are not, ( called **London( dispersion ) Forces )**
* Easy to separate; thus graphite is a good lubricant, and ideal for pencils --> writing causes sheets to fall, leaving a mark
3. **Graphene**
* Single sheet of graphite
* Thus 1 atom thick ( therefore said to be **2D)**
* Good Electrical Conductor
* Flexible
* Lightweight
* transparent
* Very strong
* Applications huge
* Bendable electronic displays & desalination technologies
4. **Fullerenes**
* Groups of carbon allotropes arranged in interlinking hexagonal and pentagonal rings
* Some form long hollow cylinders ( **Carbon Tubes )**
* Used in nanotechnology ( use of objects with dimesions of $$< 100$$nm ( 1000 atoms or less across )
* Applications huge
* Used to reinforce composite materials ( combination of two materials with different physical and chemical properties )
* Strong due to the covalent bonds
* electronics
* Presence of delocalised electrons
* Conductive
5. **Buckminsterfullerenes ( buckyballs )**
* Covalent Molecular structure
* $$C\_{60}$$
* Atoms are arranged in hexagons and pentagons to resemble a football
* Applications huge
* Medicine as drug carriers
* Low boiling point ( why ? )
**Silicon and Silicon Dioxide**
* Silicon forms 3 dimesional lattice
* Each atom --> 4 other atoms in tetahedral arrangement
* SImialr to Diamond
* Note Si-Si bond is weaker than its C-C counterpart
* Despite belonging to same group they are different
* Silicon is a metalloid, carbon is non-metal
* Thus diamond is poor electrical conductor
* Silicon is semiconductor
* Double & triple bonds common in C, not in Si
* Note that Si-O is very strong
* **Silicon Dioxide / Silica** ( $$SiO\_2$$)
* Quartz ( crystalline form )
* sand
* glass
