sio2 structure.
Carbon and silicon have four electrons in their valence shells. They are both tetravalent. It means that the structure of CO2 and SiO2 should be similar. But the structure of SiO2 and CO2 are quite different from each other.
Difference of structures of CO2 and SiO2:
In CO2, carbon makes double bond with each oxygen atom and the molecule is linear triatomic.
O = C = O
Each molecule of CO2 is a discrete unit. For this reason CO2 exists in the form of a gas. The molecule of CO2 is non-polar.
Reason for giant structure of SiO2:
- In case of SiO2, the size of the silicon atom is much larger than carbon atom. Greater numbers of oxygen atoms can surround silicon atom. In other words greater number of oxygen atoms can become the neighbours of the silicon atoms.
- Silicon can make the single bonds with oxygen atom. While in the case of CO2 double bonds are present between carbon and oxygen.
- CO2 molecule is linear. But when four oxygen atoms link with one silicon atom then a tetrahedral structure is produced. This tetrahedral structure continues in three dimensions. In this way a continuous, giant silicon oxygen network extends and a huge crystal of So2 is produced. The shape of this giant molecule is as follows: Fig. (1.1)
Some important points about the structure of SiO2:
- All silicon atom is bonded to four oxygen atoms.
- Every silicon atom is Sp3-hybridized.
- All the bond angles around silicon atom are 109.5°
- Every oxygen atom is Sp3 hybridized and has two lone pairs on it.
- It is a three dimensional network structure.
- The overall ratio of silicon to oxygen atoms is 1 : 2.
- The simplest formula of silica is SiO2.
- It is a macromolecule.
- The structure is very hard and the substance has very high melting and boiling point.
- This whole chunk of silica must be considered as one molecule.
- The atoms of silicon and oxygen at the surface of the chunk do not have their all valencies satisfied. For this reason silica has high surface activity.
- There are different polymorphic species of silica. In each of various crystalline forms of silica, there is a special pattern which is repeated throughout the crystal.
- There is a regular tetrahedral arrangement of four oxygen atoms around each silicon in each crystalline form. But Si – 0 – Si bond angles and the rotation of the bonds are difficult.
Silica glass (fused quartz)
When crystalline silica is heated, it melts. In this way it gives a viscous liquid having a random structure. In this random structure the atoms of silicon are close to four oxygen atoms and each oxygen atom is close to two silicon atoms.
When this liquid silica is cooled it is not crystallized readily. It undercooles and finely becomes rigid without having proper orientations of silica and oxygen atoms.
Definition of silica glass:
This rigid, highly undercooled liquid is called vitrous silica or silica glass. It is also called fused quartz.
