Part 4 Covalent Bonding
- giant covalent structures and polymers
Brown's Science-Chemistry Chemical Bonding GCSE/IGCSE/AS/A2 O Level Revision Notes
This section describes how covalent bonds can lead to large linear ('1D') e.g.
thermoplastic polymer macromolecules, two dimensional ('2D') structures like
graphite layers and three dimensional ('3D') giant covalent structured molecules
like diamond, silica and thermosetting plastics. The physical properties of
these structures are described and explained using models of their molecular
Part 1 Introduction - why do atoms bond together? (I
suggest you read 1st)
Ionic Bonding - compounds and properties
Covalent Bonding - small simple molecules and properties
Covalent Bonding - macromolecules and giant covalent structures
Metallic Bonding - structure and properties of metals
Part 6 More advanced concepts for
advanced level chemistry (in preparation, BUT a lot on
intermolecular forces in Equilibria Part 8)
COVALENT BONDING - macromolecules &
giant covalent structures
giant network bonding - giant molecules e.g. carbon
C-diamond/graphite, silicon Si/silica SiO2
properties of giant covalent structures *
* properties of polymers
(diamond), carbon (graphite), carbon
(buckminsterfullerene/fullerenes), silica/silicon dioxide
BIG!4. Large Covalent Molecules and their Properties
macromolecules - giant covalent networks and polymers
What is the bonding, structure
and properties of the carbon allotropes diamond, graphite &
buckminsterfullerenes (fullerenes), silica (silicon dioxide), thermosets,
Because covalent bonds act in a particular
direction i.e. along the 'line' between the two nuclei of the atoms bonded
together in an individual bond, strong structures can be formed, especially if
the covalent bonds are arranged in a strong three dimensional giant covalent
The structure of the
three allotropes of carbon (diamond, graphite and fullerenes), silicon
and silicon dioxide (silica)
- It is possible for many atoms to link up to form a giant covalent structure
or lattice. The atoms are usually non-metals.
- This produces a very strong 3-dimensional covalent bond
network or lattice.
- This gives
them significantly different properties from the small simple
covalent molecules mentioned above.
- This is illustrated by carbon in the form of
diamond (an allotrope of
carbon). Carbon has four outer electrons that form four single bonds, so each carbon
bonds to four others by electron pairing/sharing. Pure silicon, another element in
Group 4, has a similar structure.
- NOTE: Allotropes are
different forms of the same element in the same physical state.
They occur due to different bonding arrangements and so diamond,
and fullerenes (below)
are the three solid allotropes of the
- Oxygen (dioxygen), O2,
and ozone (trioxygen), O3, are the two small
gaseous allotrope molecules of the element oxygen.
- Sulphur has three solid
allotropes, two different crystalline forms based on small S8
molecules called rhombic and monoclinic sulphur and a 3rd
form of long chain ( -S-S-S- etc.) molecules called plastic
PROPERTIES of GIANT COVALENT STRUCTURES
- This type of giant covalent structure is thermally very stable and
has a very high melting and boiling points because of the
strong covalent bond network (3D or 2D in the case of graphite
- A relatively large amount of
energy is needed to melt or boil giant covalent structures. Energy changes
for the physical changes of state of melting and boiling for a range
of differently bonded substances are compared in a section of
the Energetics Notes.
- They are usually poor conductors of electricity because the electrons are not usually free to move as they can in metallic structures.
- Also because of the strength of the bonding
in all directions in the structure, they are often very hard,
strong and will not dissolve in solvents like water. The
bonding network is too strong to allow the atoms to become
surrounded by solvent molecules
- Silicon dioxide (silica, SiO2)
has a similar 3D structure and properties to carbon (diamond) shown
- The hardness of diamond enables it to
be used as the 'leading edge' on cutting tools.
- Energy changes for the physical changes of state
of melting and boiling for a range of differently bonded substances is
given in a section of
the Energetics Notes.
- Many naturally occurring
minerals are based on -O-X-O- linked 3D structures where X is often
silicon (Si) and aluminium (Al), three of the most abundant elements
in the earth's crust.
- Silicon dioxide is found as
quartz in granite (igneous rock) and is the main component in
sandstone - which is a sedimentary rock formed the compressed
erosion products of igneous rocks.
- Many some minerals that are
hard wearing, rare and attractive when polished, hold great value as
- Carbon also occurs in the form of
graphite. The carbon atoms form joined hexagonal rings forming
layers 1 atom thick.
- There are three strong covalent bonds per
carbon (3 C-C bonds in a planar
arrangement from 3 of its 4 outer
BUT, the fourth outer electron is 'delocalised'
or shared between the carbon atoms to form the equivalent of a 4th
bond per carbon atom (this situation requires advanced level concepts to fully explain,
this bonding situation also occurs in fullerenes described below,
and in aromatic compounds you deal with at advanced level).
- The layers are only held together by
weak intermolecular forces shown by the dotted lines NOT by strong
- Like diamond and silica (above) the
large molecules of the layer ensure graphite has typically very
high melting point because of the strong 2D bonding network
(note: NOT 3D network)..
- Graphite will not dissolve in solvents
because of the strong bonding
- BUT there
are two crucial differences compared to
- Electrons, from the 'shared
bond', can move freely through each layer, so graphite is a
conductor like a metal (diamond is an electrical insulator
and a poor heat conductor). Graphite is used in electrical
contacts e.g. electrodes in electrolysis.
- The weak forces enable the
layers to slip over each other so where as diamond is hard
material graphite is a 'soft' crystal, it feels slippery.
Graphite is used as a lubricant.
- These two different characteristics
described above are put to a common use with the electrical contacts in electric
motors and dynamos. These contacts (called brushes) are made of
graphite sprung onto the spinning brass contacts of the armature.
The graphite brushes provide good
electrical contact and are self-lubricating as the carbon layers
slide over each other.
- A 3rd form of carbon are
fullerenes or 'bucky balls'! It consists of hexagonal rings like
graphite and alternating pentagonal rings to allow curvature of the
- Buckminster Fullerene C60
is shown and the bonds form a pattern like a soccer ball. Others are
oval shaped like a rugby ball. It is a black solid insoluble in
- They are
considered giant covalent
structures and are classed as simple molecules. They do dissolve in
organic solvents giving coloured solutions (e.g. deep red in petrol
hydrocarbons, and although solid, their melting points are not that
- They are mentioned here to
illustrate the different forms of carbon AND they can be
made into continuous tubes to form very strong fibres of 'pipe like'
molecules called 'nanotubes'. These 'molecular size'
particles behave quite differently to a bulk carbon material like
- Uses of Nanotubes - carbon
nanotechnology - examples of nanochemistry
- They can be used as
semiconductors in electrical circuits.
- They act as a component
of industrial catalysts for certain reactions whose economic
efficiency is of great importance (time = money in business!).
- The catalyst can be attached
to the nanotubes which have a huge surface are per mass of catalyst
- They large surface combined
with the catalyst ensure two rates of reaction factors work in
harmony to increase the speed of an industrial reaction so making
the process more efficient and more economic.
- Nanotube fibres are very
strong and so they are used in 'composite materials' e.g.
reinforcing graphite in carbon fibre tennis rackets.
- Nanotubes can 'cage'
other molecules and can be used as a means of delivering drugs
in controlled way to the body because the thin carbon nanotubes can
penetrate cell walls.
- I've written NEW pages with more
examples and details on
in polymers and 1-3 'dimension' concepts in macromolecules
The bonding in polymers or plastics
is no different in principle to the examples described above, but there is quite
a range of properties and the difference between simple covalent and giant
covalent molecules can get a bit 'blurred'.
Bonds between atoms in
molecules, e.g. C-C, are called intra-molecular bonds.
The much weaker electrical
attractions between individual molecules are called inter-molecular
In thermosoftening plastics
like poly(ethene) the bonding is like ethane except there are lots of
carbon atoms linked together to form long chains. They are moderately strong
materials but tend to soften on heating and are not usually very soluble in
solvents. The structure is basically a linear
1 dimensional strong bonding networks. The polymer molecules are held
together by weak intermolecular forces and NOT strong chemical bonds. The
long polymer molecules mean the intermolecular forces are appreciable but
the material is flexible and softens on heating.
structure is a layered 2 dimensional strong bond network made of layers
of joined hexagonal rings of carbon atoms with weak
inter-molecular forces between the layers. (more
details on graphite)
plastic structures like
melamine have a 3 dimensional cross-linked
giant covalent structure network similar to diamond
or silica in
principle, but rather more complex and chaotic! Because of the strong 3D
covalent bond network they do not dissolve in any solvents and
do not soften and melt on heating and are much stronger than thermoplastics.
More on polymers in Oil
Notes and Extra
Organic Chemistry Notes.
A couple of Advanced Level scribbles',
yet to be typed up!
keywords-phrases formulae: giant covalent lattice structures SiO2
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