Giant Covalent Structures
A giant covalent structure is a three-dimensional structure of atoms that are joined by covalent bonds. Allotropes are different forms of the same element, in the same state. Graphite ,graphene and diamond are allotropes of the same element (carbon) in the same state (solid). Carbon can form up to four covalent bonds.
Diamond and graphite are allotropes of carbon which have giant covalent structures
This class of substances contains a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds forming a giant lattice structure
Giant covalent structures have high melting and boiling points as they have many strong covalent bonds that need to be broken down
Large amounts of heat energy are needed to overcome these forces and break down bonds
Uses of Giant Covalent Structures
Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure. As a result, diamond is very hard and has a high melting point. This explains why it is used in cutting tools. It does not conduct electricity.
Diamond
Each carbon atom bonds with four other carbons, forming a tetrahedron
All the covalent bonds are identical and strong with no weak intermolecular forces
Diamond thus:
1) Does not conduct electricity
2) Has a very high melting point
3) Is extremely hard and dense (3.51 g/cm3)
4) Diamond is used in jewellery and as cutting tools
5) The cutting edges of discs used to cut bricks and concrete are tipped with diamonds
6) Heavy-duty drill bits and tooling equipment are also diamond-tipped
Graphite
Each carbon atom is bonded to three others forming layers of hexagonal-shaped forms, leaving one free electron per carbon atom
These free electrons exist in between the layers and are free to move and carry charge, hence graphite can conduct electricity
The covalent bonds within the layers are very strong but the layers are connected to each other by weak intermolecular forces only, hence the layers can slide over each other making graphite slippery and smooth
Graphite thus:
1) Conducts electricity
2) Has a very high melting point
3) Is soft and slippery, less dense than diamond (2.25 g/cm3)
4) Graphite is used in pencils and as an industrial lubricant, in engines and in locks
5) It is also used to make non-reactive electrodes for electrolysis
The Structure of Silicon(IV) Oxide (Silicon Dioxide)
SiO2 is a macromolecular compound which occurs naturally as sand and quartz
Each oxygen atom forms covalent bonds with 2 silicon atoms and each silicon atom in turn forms covalent bonds with 4 oxygen atoms
A tetrahedron is formed with one silicon atom and four oxygen atoms, similar as in diamond
Diamond & Silicon(IV) Properties
Silica (or silicon dioxide), which is found in sand, has a similar structure to diamond, so its properties are similar to diamond. It is hard and has a high melting point, but contains silicon and oxygen atoms, instead of carbon atoms.
SiO2 has lots of very strong covalent bonds and no intermolecular forces so it has similar properties to diamond
It is very hard, has a very high boiling point, is insoluble in water and does not conduct electricity
SiO2 is cheap since it is available naturally and is used to make sandpaper and to line the inside of furnaces
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