Silicon Dioxide - SiO2

  • Silicon dioxide is a 3D (three dimentional) network type of solid. The Si-O bonds are strong covalent bonds. The strength of the covalent bonds depend on the electronegativity of the atoms involved in the covalent bonding. Electronegativity is the pull of electrons shared in a covalent bond by atoms. In silicon dioxide the O (oxygen) atom has a stronger pull on the electrons than silicon, meaning it has a strong electronegativity and that the electrons shared in the covalent bond spend more time around the oxygen atom.

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Melting Point:
Silicon dioxide has a large 3D Covalent Network. The covalent bond between the Si-O atoms are very strong. These covalently bonded molecules have weak Van der Waals intermolecular forces between them that can be easily broken. The temperature at which silicon dioxide melts at 1700 degrees celcius. This tells us that silicon dioxide has a high melting point as it requires a lot of heat energy to break thorugh the collection of intermolecular forces and strong covalent bonds.
  • The weak intermolecular forces are easily broken, so therefore require less heat energy to break the forces. Because silicon dioxide has a 3D network of the solid silicon dioxide, the intermolecular forces are repeated in the layers, making the forces voluminous (meaning bigger and larger) needing more heat energy to break all the weak intermolecular forces. Once these are broken, the covalent bonds (intramolecular, beacuse they are within the molecules atoms) are heated so they seperate and the substance can change its state - melt.

  • Due to the strong covalent bonds, the SiO2 solide is very hard, as its held strongly together by 3D network. The silicon and oxygen atoms are held in fast(strong and close) 3D crystals. The layering of the 3D network structure also contributes ro the hardness of the substance as well as the strong covalent bonds between the atoms of the SiO2 molecule

Electrical Conductivity:

In the SiO2 solid, there are no delocalised sea of electrons (free moving electrons) as they are held tightly between the silicon and oxygen atoms. Because of this reason SiO2 is unable to conduct electricity. However, in molten state, SiO2 can conduct elctricity as the silcon and oxygen atoms are free moving and negatively charged which allows them to carry electrical charge.

  • Silicon dioxide is insoluble in water and any other solvents. This is becuase there is no attraction between polar solvent molecules and the silicon or oxygen atoms to overcome the covalent bonds. The large crystal size and the non-polarity of the SiO2 stops the substance from dissolving.


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