The Structure and Properties of Solids
Covalent Network Crystals(Section 4.6 pages 270 - 273 in text)
Covalent crystals, also called network solids or macromolecules, consist of atoms of the same or different elements joined together by a network of single covalent bonds.
This network of bonds can exist in ______to produce a variety of properties
Allotropes: ______
______
For example graphite and diamond are allotropes of carbon (as is “Buckeyball”).
Both form carbon dioxide and water when undergoing combustion but only graphite conducts electricity and diamond is one of the hardest substances known.
3-D network – examples: carbon as diamond, quartz, silicon carbide
These network solids consist of covalently bonded atoms which form regular 3-D arrays or crystals.
Much like an ionic crystal, the intermolecular bonds are the same as the intramolecular bonds (covalent bonds).
Due to the very strong bonds, these solids will have:
- very high melting and boiling points
- will be solids at room temperature
- be extremely hard
- not soluble in polar or nonpolar solvents
- do not conduct electricity.
In diamond, we see atoms of carbon covalently bonded to four other carbon atoms in a rigid tetrahedral pattern. This network of strong bonds makes diamond the hardest known substance, with a melting point and boiling point of 3550°C and 4827°C respectively
3-dimensional array of carbon atoms covalently bonded in a tetrahedral arrangement.
- Strong covalent bonds give diamond its hardness.
- There are no delocalized electrons, therefore diamond will not conduct electricity.
- There are several planes of atoms within the crystal and diamonds are cut along these planes. Light is reflected by these planes to give diamonds their sparkle (total internal refraction)
Network Solids of Silicon
•Silicon makes up 25.7 % of the Earth’s crust.
Quartz
A large 3-dimensional network with silicon and oxygen.
Each silicon is a tetrahedral bond to four oxygen atoms.
Foreign metal ions in quartz produce semiprecious stones such as emerald, amethyst and garnet.
2-D network – example: carbon as graphite, mica
Networks which form ______.
The properties of graphite are quite different from those of diamond.
While the melting point and boiling point are still very high, graphite is ______
______.
The layers will slide over each other allowing them to be used as a lubricant.
Each carbon atom is covalently bonded to only 3 other carbon atoms to form flat sheets or layers with weak ______existing between the layers.
The electrons making up the “double bonds”are actually delocalized throughout the structure (i.e. not true double bonds). The layers of hexagons are held in place.
The delocalized electrons are able to move freely therefore allowing graphite to conduct electricity.
When you write with a pencil you are breaking of layers of graphite
Graphite – side view
Graphite – side view showing p orbitalsGraphite – side view showing formation of π bonds
1-D network – example: asbestos
These are solids that form networks in a one dimensional array or fibre.
- They consist of long chains held together by covalent bonds.
- The forces between adjacent chains are very weak therefore the solids will form threads.
- They have very high melting and boiling points due to the strong covalent bonds.
- They are solids at room temperature and are not soluble in water
Asbestos is composed of silicon, oxygen and certain metallic elements such as magnesium and calcium.
The fibrous nature and other properties of this mineral lead us to conclude that it consists of a one-dimensional network of bonds.
The silicon and oxygen atoms form long chains with strong covalent bonds between the atoms.
However the force between adjacent chains is very weak, resulting in a substance with a thread-like structure.