Giant+Covalent+structures

These are covalent substances which consist of many atoms covalently bonded to each other to form one giant molecule. Therefore there are no molecules as such and even if there are they are very big consisting of many atoms (60 atoms in the case of fullerene) Some good example of giant covalent structures are the allotropes of carbon (diamond, graphite and fullerene) and silicon dioxide in sand.

//**Silicon Dioxide (SiO 2 ) **// This is a tetrahedral structure consisting of one silicon atom bonded to 4 other oxygen atoms and each oxygen atoms bonded to 2 other silicon atoms. 

**//The allotropes of Carbon //** Carbon exhibits allotropy which means that it can exist in more than one physical form (called allotropes) in the same state. Carbon for example can exist as diamond, graphite and fullerene (C 60 ) in the solid state. Each carbon atom is bonded to four other carbon atoms with equal strong covalent bonds forming a tetrahedral structure. As a result of its strong carbon-to-carbon (C-C) bonds, diamond is extremely hard and has a high melting point. There are no delocalized electrons (electrons which are free to move throughout the structure) since all electrons are used up to form covalent bonds with the neighbouring carbon atoms. As a result diamond doesn’t conduct electricity. //(Diamond has the same structure as silicon dioxide) //  Each carbon atom forms strong covalent bonds with 3 other carbon atoms (in a trigonal planar structure) linking to form a layer of hexagonal rings. There are weak forces of attraction between the layers since they are formed by delocalized electrons moving between the layers. As a result of these delocalized electrons graphite is a good conductor of electricity. The layers can easily slide over each other, thus graphite can be used as a lubricant or to make pencils.  60 carbon atoms are joined together in a combination of pentagonal and hexagonal rings forming a sphere. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Fullerene has a molecular structure, that means that it consists of molecules (unlike graphite or diamond which don’t have molecules, they are simply whole structures with many atoms bonded together—that is the reason why they are called giant covalent structures) <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Fullerene dissolves in non-polar solvents and has a low melting point.
 * <span style="color: #18bf28; font-family: Tahoma,sans-serif; font-size: 14pt;">Diamond **
 * <span style="color: #18bf28; font-family: Tahoma,sans-serif; font-size: 14pt;">Graphite **
 * <span style="color: #18bf28; font-family: Tahoma,sans-serif; font-size: 14pt;">Fullerene (C 60 ) **
 * [[image:t838_1_015i.jpg caption="Look closely at the pentagonal and hexagonal rings (similar to a football. Isn't it?"]] || [[image:c60.gif width="215" height="266" caption="Fullerene, with the carbon atoms bonded to each other in rings of 5s and 6s with a total of 60 Carbon atoms in the molecule. This is one molecule!!"]] ||


 * By the end of this lesson you should be able to: **
 * ** Describe the structure and bonding in silicon dioxide (SiO 2 ) **
 * ** Describe and compare the structure and bonding in the three allotropes of carbon (diamond, graphite and fullerene) **
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// The photos used on this page were taken from the following sources: // Silicon dioxide [] (accessed 11/04/2012) <span style="font-family: Calibri,sans-serif; font-size: 9pt;">diamond [] (accessed 11/04/2012) Graphite [] (accessed 11/04/2012) Fullerene (left) [] (accessed 11/04/2012) Fullerene (right) [] (accessed 11/04/2012)