Covalent network crystals (also referred to as “covalent crystals” or “network crystals”) consist of atoms held together in large networks or chains by covalent bonds (a covalent network crystal is essentially one large molecule).
Covalent crystals are found in three categories, depending on the orientation of bonding with the crystal lattice: 3D, 2D or 1D. We will discuss them all below:
3-D Covalent Network Solids
In diamond, each C is covalently bonded to four other carbon atoms. This 3-D structure contributes to diamond’s amazing hardness.
SiO2 (quartz) is not as hard because of different sized atoms, which results in weaker bonds.
Metalloids, like Si and Ge, are also found as covalent crystals.
Another 3-D covalent crystal is silicon carbide (SiC, known as carborundum).
Properties of 3D Covalent Network Solids
Mechanical Strength
- extremely hard (will cleave) due to very strong covalent bonds
- insulators because there are no free electrons or ions to carry electricity
Melting Point
- extremely high since strong covalent bonds are difficult to break
- usually transparent because there are no free electrons to interact with the light
2-D Covalent Network Solids
Graphite, which is based on the benzene ring (C6H6), is composed of stacked sheets consisting of interlocking benzene rings without hydrogen.
Mica
(Si2O5) is the other substance that is a 2-D covalent network crystal.
In most molecules the electrons are localized on the atom (i.e. the σ- and 𝛑-electrons are associated totally with the two atoms forming the bond). In benzene (and hence, graphite), the electrons are delocalized (the electrons are semi-mobile and can move about causing resonance structures). Therefore, graphite has (a) very strong covalent bonds linking C atoms in the plane of the sheets and (b) weak London forces between the sheets, holding them together.
Let's take a closer look at the bonding in the six carbon benzene ring to see how this works. We will focus on one of the six carbon rings that is found in the graphite sheet.
Properties of 2-D Covalent Network Solids
Mechanical Strength
- graphite is a soft solid that cleaves in planes (because the London forces are weak); can be used as a lubricant
- very
high since the covalent bonds require a large amount of energy to break
Electrical Conductivity
- excellent in graphite since it has delocalized electrons to carry the electricity
- poor in mica since there are no delocalized electrons
- opaque and shiny in graphite due to the interaction of light with the delocalized electrons
- transparent in mica since there are no delocalized electrons to interact with the light
1D Covalent Network Solid
There is only one example of a 1D covalent network solid: asbestos. Asbestos mainly consists of alternating silicon and oxygen atoms bonded to each other. Asbestos, due its 1D structure is fibrous in nature.
Allotropes of Carbon
We have already discussed diamond and graphite, but there is a third allotrope (physical form) of carbon, discovered in 1985, C60, aka, Buckminsterfullerene (or Bucky Ball for short).
Some scientists believe that carbon nanotubes (relatives of Bucky Ball) could be used to create a space elevator.
Homework # 31-37
