# 2.2.7 Covalent Network Structures

* 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.

<figure><img src="/files/rylOlO3IFyZr0XoebfCD" alt=""><figcaption><p>a- diamond b - graphite  c - graphene d- nanotubes  e- buckminsterfullerene (C_60)</p></figcaption></figure>

1. **Diamond**

* Each carbon bonded to four others in tetrahedral arrangement&#x20;
* Poor electrical conductor due to no mobile charged particles ( electrons are localized )&#x20;
* 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 )**&#x20;
  * Easy to separate; thus graphite is a good lubricant, and ideal for pencils --> writing causes sheets to fall, leaving a mark

3. **Graphene**&#x20;

* Single sheet of graphite&#x20;
  * 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&#x20;
      * Conductive

5. **Buckminsterfullerenes ( buckyballs )**&#x20;

* Covalent Molecular structure&#x20;
* $$C\_{60}$$
* Atoms are arranged in hexagons and pentagons to resemble a football
* Applications huge
  * Medicine as drug carriers
  * Low boiling point ( why ? )&#x20;

**Silicon and Silicon Dioxide**

* Silicon forms 3 dimesional lattice&#x20;
  * Each atom --> 4 other atoms in tetahedral arrangement
    * SImialr to Diamond
* Note Si-Si bond is weaker than its C-C counterpart

<figure><img src="/files/VvtAsgOiKUOP84lw6wsa" alt=""><figcaption></figcaption></figure>

* 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


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