Allotropes of Carbon
and Silicon Dioxide
Covalent Structures
Carbon in many forms…
Allotropes
O An element is said to have allotropes when it
exists in the same state in more than one
structural form.
O The allotropes of an element have the same
or similar chemical properties, but their
physical properties are different.
O There are three common allotropes of
carbon - diamond, graphite and the
fullerenes.
Carbon allotropes
Carbon allotropes
Diamond
Graphite
C60 Fullerene
Bonding
Each carbon bonded
to four other carbon
atoms. Arranged
tetrahedrally.
Each carbon bonded
to three other carbon
atoms. Arranged in
layers of hexagonal
rings.
Structure
Giant covalent
Giant covalent
Each carbon bonded
to three other carbon
atoms. Spherical
molecules made up
of 20 hexagons and
12 pentagons.
Simple covalent
sp3
sp2
sp2/sp3
Hybridization
C-C bond length/nm
0.154
0.143 (in the plane)
0.335 (between the
layers)
0.140 (between two
hexagons)
0.145 (between a
hexagon and a
pentagon)
Carbon allotropes
Diamond
Graphite
C60 Fullerene
Melting point/K
3500
3700
Sublimes at 800
Density/g cm-3
3.53
2.25
1.65
Electrical conductivity
None
Good
Poor
Very Good
Good
Poor
Insoluble in all kinds
of solvents
Very hard
Insoluble in all kinds
of solvents
Soft
Soluble in non-polar
solvents e.g. toluene
Very hard
Conduction of heat
Solubility
Hardness
Check your understanding…
O Why is diamond so much harder than
graphite?
O Diamond is so much harder than graphite
because all the carbon atoms in diamond
are held together by strong covalent bonds
in a giant lattice structure. The weak forces
of attraction between the layers of carbon
atoms in graphite are easily broken.
Check your understanding…
O Why does graphite conduct electricity while
diamond does not? Why is graphite a better
conductor than C60?
O The presence of a cloud of free/delocalised
electrons in between layers of carbon atoms in
graphite enables it to conduct electricity. All the
electrons in the carbon atoms present in
diamond are involved in the formation of strong
covalent bonds and thus there are no free
electrons available to conduct electricity.
Check your understanding…
O Compare the melting points of diamond,
graphite and C60.
O Graphite has the highest melting point
(3700 oC) because a lot of energy is needed
to break the strong C-C bonds (sp2/ bond
length 0.143 nm) between the carbon atoms
in graphite.
Check your understanding…
O Diamond also had a high melting point
(3500 oC) because a lot of energy is needed
to break the strong C-C bonds (sp3/ bond
length 0.154 nm) between the carbon
atoms in diamond.
O C60 fullerene sublimes at 800O C as it still
consists of molecules, though the molecules
are large. Only weak intermolecular forces
are needed to be overcome.
Check your understanding…
O Why is graphite used in lead pencils and
also as a lubricant?
O Graphite is used in lead pencil and as a
lubricant because it consists of layers of
carbon atoms that are held weakly together
and thus can slide over one another easily
when a force is applied.
Check your understanding…
O Why are diamond and graphite almost totally
insoluble in all kinds of solvent? How would
you explain the solubility of C60 in non-polar
solvents?
O Diamond and graphite are almost totally
insoluble in all kinds of solvent because of
their giant covalent structures. C60 fullerene
is soluble in non-polar solvents because it
consists of molecules (though the molecules
are large).
Silicon and silicon dioxide
Silicon and silicon dioxide
O Silicon and silicon dioxide have structures
similar to diamond. In silicon dioxide, each
silicon is bonded to four oxygen atoms and
each oxygen atom is bonded to two silicon
atoms. The properties are similar to
diamond.