Silicon dioxide
Has a high melting point – varying depending on what the particular structure is (remember that the structure given is only one of three possible structures), but around 1700°C.
Very strong silicon-oxygen covalent bonds have to be broken throughout the structure before melting occurs.
- Is hard. This is due to the need to break the very strong covalent bonds.
- Doesn’t conduct electricity. There aren’t any delocalized electrons. All the electrons are held tightly between the atoms, and aren’t free to move.
- Is insoluble in water and organic solvents.
There are no possible attractions which could occur between solvent molecules and the silicon or oxygen atoms which could overcome the covalent bonds in the giant structure.
Uses of Silica
- Quartz glass is used for manufacturing optical instruments.
- Colored quartz is used for manufacturing gems.
- Sand is used in manufacture of glass, porcelain, sand paper and mortar etc.
- Sand stone is used as a building material.
Co-ordinate (dative covalent) bonding
A covalent bond is formed by two atoms sharing a pair of electrons. The atoms are held together because the electron pair is attracted by both of the nuclei. In the formation of a simple covalent bond, each atom supplies one electron to the bond – but that doesn’t have to be the case.
A co-ordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.
The reaction between ammonia and hydrogen chloride
If these colourless gases are allowed to mix, a thick white smoke of solid ammonium chloride is formed. Ammonium ions, NH4+, are formed by the transfer of a hydrogen ion from the hydrogen chloride to the lone pair of electrons on the ammonia molecule.
When the ammonium ion, NH4+, is formed, the fourth hydrogen is attached by a dative covalent bond, because only the hydrogen’s nucleus is transferred from the chlorine to the nitrogen. The hydrogen’s electron is left behind on the chlorine to form a negative chloride ion.
Once the ammonium ion has been formed it is impossible to tell any difference between the dative covalent and the ordinary covalent bonds. Although the electrons are shown differently in the diagram, there is no difference between them in reality.