4d. Silicon and silicon dioxide giant covalent structures

Also, a comparison of the structure and physical properties of carbon dioxide and silicon dioxide

Doc Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A level US grade 9-12 level Revision Notes


  • Pure elemental SILICON (not the oxide) has the same molecular structure as diamond and similar properties, though the 3D giant covalent bond network is not as strong, so elemental silicon is not as high melting as carbon in the form of diamond.
    • The molecular diagram is the same for Sin, where n is a huge number!
      • (Its the same ball and stick diagram I used for diamond, but the atoms would be a bit bigger)
    • Silicon melts at 1400oC, and has poor electrical conductivity (no free electrons or ions) and won't dissolve in any solvent.
    • The silicon in the transistors of electronic devices is 'doped' with other elements to increase its electrical conductivity.

  • SILICON DIOXIDE (SILICA) (SiO2)n where n is an extremely large number of silicon and oxygen atoms!
    • Many naturally occurring minerals are based on OXO linked 3D structures where X is often silicon (Si) and aluminium (Al), three of the most abundant elements in the earth's crust.
    • Silicon dioxide ('silica') is found as quartz in granite (igneous rock) and is the main component in sandstone which is a sedimentary rock formed the compressed erosion products of igneous rocks.
      • (c) doc bLooking at the diagram on the right of silica, each silicon atom (black blobs) forms four strong covalent bonds with the linking oxygen atoms (yellow blobs).
      • You can also see from the diagram that there are two oxygen atoms to every silicon atoms giving the empirical formula SiO2.
      • Again like diamond, theoretically all the atoms in a silica crystal are linked together by a strong 3D covalent bond network giving a strong rigid structure.
      • It takes a lot of energy to break (overcome) the strong silicon-oxygen bonds in the giant covalent lattice of silicon dioxide (silica), hence the high melting point of 1600oC.
    • Therefore Silica (SiO2) is a very hard substance with a very high melting point and won't dissolve in any solvent.
    • There are no free electrons so silicon dioxide doesn't conduct electricity.
    • Many more minerals that are hard wearing, rare and attractive when polished, hold great value as gemstones, but sand is also mainly silica, but not quite as valuable!

A comparison of carbon dioxide and silicon dioxide

Carbon and silicon are in the same group IV, they have the same oxide formula,

BUT some very different physical properties.

 Its all about the nature of the bonding situation!

Property Carbon dioxide, CO2 Silicon dioxide, SiO2
Structure Small individual covalently molecules

O=C=O  O=C=O  O=C=O

Strong covalent double bonds between the carbon and oxygen atoms within the molecule itself, BUT weak intermolecular forces between the molecules

(c) doc b3D giant covalent network of strong C-O-C bonds
Physical state at room

 

GAS

Sublimes at -78oC, because of the weak intermolecular forces

SOLID

Melts at 1710oC, , because of the very strong inter-atomic forces - much higher thermal kinetic energy needed to break down the structure

Physical strength Weak brittle solid below -78oC, because of the weak intermolecular forces Very strong hard material.
Solubility Will dissolve in many solvents e.g. water or organic liquids. The solvent can overcome the weak intermolecular forces to dissolve the molecules. Will NOT dissolve in any solvents like water or organic liquids. The solvent cannot overcome the strong covalent bonds to dissolve the giant molecules.
Conductivity The solid crystal is a poor conductor of thermal energy and electricity The solid crystal is a poor conductor of thermal energy and electricity

You can also compare the similarity of carbon (diamond), silicon Si and silicon dioxide SiO2 - the bonding diagrams say it all.

All three have a similar 3D giant covalent structure, like the one above or on the right, which could be carbon (diamond) or the element silicon.

They will therefore share a similar set of physical properties described above for silicon dioxide.


What next?

Recommend next: Covalent bonding in polymers and 1, 2 and 3 'dimension' concepts in macromolecules

 

Sub-index for: Part 4 Giant covalent structures and other big molecules

 

Index for ALL chemical bonding and structure notes

 

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