4a. Introduction to giant covalent structures
Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A
level US grade
9-12 level Revision Notes
Large 3D 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)?, why does diamond have such a high melting
point? why is silica (silicon dioxide) a giant covalent structure, thermosets,
thermoplastics? 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
Its a good idea to have some idea
of where the elements forming giant covalent structures are in the periodic table
The black zig–zag line 'roughly' divides the metals
on the left from the non–metals on the right of the elements of the Periodic
Part of the modern Periodic Table
Pd = period,
Gp = group
metals => non–metals
that H does not readily fit into any group
Chemical Symbol eg 4Be
1 Alkali Metals
Gp 2 Alkaline Earth Metals
Gp 7 Halogens
Gp 0 Noble Gases
Chemical bonding comments about the
selected elements highlighted in white
The non–metallic elements carbon and
silicon form giant covalent structures
Materials that consist of giant covalent structures
are solids with very high melting points and usually physically hard
materials (not graphite).
All of the atoms in these structures are linked to
other atoms by strong covalent bonds in specific directions eg a grain of
sand (silica) is one giant molecule!
substances usually have an extended 3D network of strong covalent bonds.
These strong bond networks must be overcome to melt or boil these giant covalent substances
and this requires very high temperatures to give the particles sufficient
kinetic energy to weaken the bonds and cause melting of the substance.
Diamond and graphite (forms of carbon), silicon
and silicon dioxide (silica) are examples of giant covalent structures.
should be able to recognise giant covalent structures from diagrams showing
their bonding and structure.
You also need to be able to explain the
properties of giant covalent substances in terms of their molecular
Most giant covalent structures don't have freely moving charged
particles like ions or electrons to carry an electric current, so they are poor conductors of
- It is possible for many atoms to link up to form a giant covalent structure
- The structures of giant covalent
structure are usually based on non–metal atoms like carbon, silicon
- The atoms in a giant covalent
lattice are held together by strong directional covalent bonds and
every atoms is connected to at least 2, 3 or 4 atoms.
- What you might call 'atomic
- This very strong 3–dimensional covalent bond
network or lattice gives the structure great thermal
stability e.g. very high melting point and often great
- This is because it takes so much
thermal kinetic energy to weaken the bonds sufficiently to allow
- The covalent bonds are very directional,
giving rise to a strong and fixed network that we call a giant
- This gives
them significantly different properties from the small simple
covalent molecules (see simple molecular
- 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,
are the four solid allotropes of the
element carbon. Liquid fullerenes are a 5th form of carbon.
- 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
- A relatively large amount of
energy is needed to melt or boil giant covalent structures because
strong chemical bonds must be broken (and not just weakening
intermolecular forces as in the case of small covalent molecules
- 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
are in metallic structures (and
giant covalent structures are NOT made up of ions).
- All the valency bonding electrons are
tightly held and shared by the two atoms of any bond, so in giant
covalent structures they are rarely free to move through the lattice
and not even when molten either, since these giant molecular
covalent structures do NOT contain ions.
- 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
below and also pure silicon itself.
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Diamond, graphite and graphene - three
allotropes of carbon
Giant covalent structures and other big
bonding and structure notes
Perhaps of interest?
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Nanochemistry (index of pages)
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