SCH3UB
Date ___________________________
4.2.9 Describe and compare the structure and bonding in the three allotropes of carbon (diamond,
graphite and C60 fullerene).
4.2.10 Describe the structure of and bonding in silicon and silicon dioxide.
Giant Covalent Network Molecules
These are giant molecular lattice structures. This implies that strong covalent bonding holds their atoms
together in a highly regular extended network. The bonding between the atoms goes on and on in three
dimensions.
Recall that melting requires the separation of the molecules comprising the solid state, and boiling
requires the separation of the molecules comprising the liquid state. Because of the large amount of
energy needed to break huge numbers of covalent bonds, all giant covalent network structures have
high melting points and boiling points and are insoluble in water. Diamond, graphite (allotropes of
carbon), fullerene (C60) and quartz (silicon(IV) oxide, SiO2) are examples.
ALLOTROPES OF CARBON: Diamond, Graphite and Fullerene
ALLOTROPES occur when an element exists in different crystalline forms. Diamond, graphite and
fullerene are all different crystalline forms of the element carbon, and are referred to as “allotropes”.
In DIAMONDs,
 Each carbon atom is COVALENTLY bonded to 4 other C atoms in a tetrahedral arrangement (bond angle
109.50) to form a GIANT COVALENT STRUCTURE.
 All of the covalent C-C bonds are equally strong so diamond is exceptionally hard. Indeed, diamond is the
hardest known natural substance.
 A very high melting point (3800 °C).
 It is used for jewellery, drill bits for drilling through rock and in cutting glass.
 There are no “delocalized” electrons (ie, electrons that do not have a specific position in a bond) in
diamond, so it is unable to conduct electricity.
 The model below illustrates diamond’s immensely strong, rigid structure
In GRAPHITE
 Each C atom is covalently bonded to 3 other C atoms in the same plane.
 The bond angle is 120°, and so the carbon atoms can form six-membered rings that link
up to form planes or flat sheets of carbon atoms.
 Between the sheets of hexagonal rings of C, delocalized electrons that are
simultaneously attracted to the carbon nuclei of the layers above and below it, keep the
layers weakly in place.
 Because of the weak “bonding” or attractive forces that keep the layers in place, the
layers can slide over each other, so graphite makes an excellent lubricant.
 The delocalized electrons also allow graphite to conduct electricity, because these
electrons are free to move
1
SCH3UB
Date ___________________________
www.drbateman.com
(BUCKMINSTER)FULLERENE, C60, is a third allotrope of carbon.
o
It consists of 60 carbon atoms joined together in hexagons and pentagons to give a geodesic spherical
structure similar to a soccer ball.
o
Many other variations of fullerene have been discovered, where sheets of carbon atoms are rolled into
tubes, etc (eg nanotubes). This has given rise to a new branch of science called nanotechnology.
www.nanoprintech.com
See http://www.avogadro.co.uk/structure/chemstruc/molecular/molecular.htm
Recently, a single layer of a graphite sample was isolated and given the name “graphene”. It is expected to be
useful in electronics and optics.
“The 2010 physics prize was awarded for producing, isolating, identifying and characterising graphene, a single
atomic layer of carbon whose unique properties make the material attractive for electronic applications.
Scientists at the University of Manchester were recognized for their work on graphene sheets peeled from blocks
of graphite.”
http://www.domain-b.com/technology/2010/20101022_researchers.html
2
SCH3UB
Date ___________________________
SILICON
SILICON
 Exists in a crystal lattice structure identical
to that of diamond, where every silicon atom
is bonded to 4 other silicon atoms in a
tetrahedral arrangement
 Like diamond, it is very hard and has a high
melting point.
 Unlike diamond, the electrons involved in
the covalent bonds are not held as tightly,
and once the lattice is formed, these electrons
are somewhat mobile, making silicon useful
as a semi-conductor of electricity for
electronic applications such as silicon chips
in computers.
SILICON DIOXIDE (empirical formula SiO2)
o
o
o
o
cnx.org
Each silicon atom can bond to 4oxygen atoms, and this gives rise to a giant covalent network
structure in which each Si is bonded to four oxygens and each O to two silicon atoms.
Again, the bonding between the atoms goes on and on in three dimensions.
Silicon dioxide is insoluble in water and has a very high melting point (1710 °C) due to its strong covalent
bonds which must be broken in order to melt or dissolve
It is the component of quartz and sand – used in the glass industry
www.keletanert.hu
Good website for viewing graphite and diamond structure silicon dioxide’s giant covalent network in 3D
www.avogadro.co.uk/structure/chemstruc/network/g-molecular.htm
For very good note and diagrams explaining giant covalent networks of diamond, graphite and silicon
dioxide go to
http://www.chemguide.co.uk/atoms/structures/giantcov.html
3
SCH3UB
Date ___________________________
4