The structure of silicon(IV) oxide
Definition
Giant covalent structure
A three-dimensional network of a very large number of atoms, each joined to its neighbours by strong covalent bonds.
- Silicon(IV) oxide, SiO2\text{SiO}_2SiO2, is also called silica and is the main compound in sand and in the mineral quartz.
- Silicon(IV) oxide has a giant covalent structure, in which silicon and oxygen atoms are joined by strong covalent bonds in a huge three-dimensional network.
- Each silicon atom is covalently bonded to four oxygen atoms, with the four bonds pointing towards the corners of a tetrahedron.
- Each oxygen atom is covalently bonded to two silicon atoms, so it links one tetrahedron to the next.
- There are therefore twice as many oxygen atoms as silicon atoms in the network, which is why the formula is SiO2\text{SiO}_2SiO2.
- The arrangement is very similar to diamond, except that an oxygen atom sits between every pair of neighbouring silicon atoms.
- Like diamond, silicon(IV) oxide contains no separate molecules, so its formula shows only the ratio of atoms in the giant structure.
Similarities with diamond
- Diamond and silicon(IV) oxide have similar properties, because both have giant covalent structures in which every atom is joined to its neighbours by strong covalent bonds in all three dimensions.
- Both have very high melting points and boiling points, because a very large number of strong covalent bonds must be broken to melt them.
- Silicon(IV) oxide melts at about 1710 ∘C1710\ ^{\circ}\text{C}1710 ∘C, and diamond does not melt until well above 3500 ∘C3500\ ^{\circ}\text{C}3500 ∘C.
- Both are very hard, because the rigid network of strong covalent bonds resists being scratched or broken.
- The hardness of silicon(IV) oxide is why sand is used to make sandpaper.
- Both are poor conductors of electricity, because all the outer-shell electrons are used in covalent bonds and there are no ions or delocalised electrons free to move.
Example
Silicon(IV) oxide and carbon dioxide
- Silicon and carbon are both in Group IV, and both form an oxide with the formula XO2\text{XO}_2XO2.
- Carbon dioxide is made of small, separate CO2\text{CO}_2CO2 molecules, held near one another only by weak intermolecular forces, so it is a gas at room temperature.
- Silicon(IV) oxide is a giant covalent structure, in which strong covalent bonds must be broken to melt it, so it is a hard solid that melts at about 1710 ∘C1710\ ^{\circ}\text{C}1710 ∘C.
- The difference in structure, not the difference in the elements, is what makes one oxide a gas and the other a hard solid.
Common Mistake
- Do not describe silicon(IV) oxide as being made of molecules, because it is a giant covalent structure with no separate molecules.
- Do not say that intermolecular forces are overcome when silicon(IV) oxide melts, because it is strong covalent bonds that must be broken.
Exam technique
Describing silicon(IV) oxide
- When describing the structure, state that it is giant covalent and that each silicon atom is bonded to four oxygen atoms while each oxygen atom is bonded to two silicon atoms.
- When comparing it with diamond, give a shared property, such as a high melting point or hardness, and explain it using many strong covalent bonds in a giant structure.
Self review
- How many oxygen atoms is each silicon atom bonded to in silicon(IV) oxide?
- How many silicon atoms is each oxygen atom bonded to?
- Why does silicon(IV) oxide have a very high melting point?
- Why does silicon(IV) oxide not conduct electricity?
- Give two properties that silicon(IV) oxide shares with diamond.
