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Structure
& bonding (Part 2)
Francine
Taylor-Campbell, Contributor
In
this week's lesson we will be comparing
the properties of ionic solids and
giant molecular crystals such as diamond
and graphite.
YOU
SHOULD BE ABLE TO:
- Explain
the term allotrophy
- Construct
diagrams to represent the structure
of sodium chloride, diamond and
graphite
- Relate
the structure to their properties
MAIN
POINTS
- Allotrophy
is the ability of an element to
exist in the same physical state
but in different structural forms.
This causes them to have different
physical properties but the same
chemical properties.
- Diamond
and graphite are giant molecular
or macromolecular crystals.
- Diamond
consists of carbon atoms tetrahedrally
arranged and bonded by strong covalent
bonds.
- Graphite
consists of carbon atoms arranged
in hexagonal rings and in layers.
- Sodium
chloride is an ionic solid having
a giant structure.
In
diamond, four carbon atoms are joined
in a tetrahedral arrangement. This
is repeated throughout to give a three-dimensional
structure with strong covalent bonds.
In
graphite, each carbon atom is bonded
to three other atoms arranged hexagonally
in layers. These layers are held together
by weak bonds which enable them to
slide over each other. There are strong
covalent bonds, however, between the
carbon atoms in each layer. Note that
for graphite, since the carbon atom
is bonded to only three others, it
means that each carbon atom has a
fourth electron not involved in bonding;
that is a free mobile electron. This
will influence the properties of graphite.
Graphite
and diamond are composed of carbon
atoms but their structures are different,
hence these solids are allotropes.
They show the same chemical properties
since they have the same element,
carbon, but the difference in their
structure causes them to have different
chemical properties.
| Property |
Sodium
Chloride |
Diamond |
Graphite |
| Appearance |
Crystalline
solid |
Sparkling
solid |
Dark
solid |
Hardness
|
Brittle
- easily split
|
Very
hard - due to strong covalent
bonds in the structure
|
Soft
and flaky - due to weak bonds
between the layers. Layers can
slip over each other hence it
is a good lubricant. |
Melting
point
|
High
due to strong ionic bonds that
need a lot of energy to break
|
Very
high due to strong covalent bonds
that need vast amounts of energy
to break |
Very
high due to strong covalent bonds
that are difficult to break. |
Electrical
Conductivity
|
Conducts
electricity when dissolved in
solution or when molten as the
ions are free to move
|
Cannot
conduct electricity because all
electrons are involved in bonding
hence there are no free electrons
to carry a current |
Conducts
electricty when solid as mobile
electrons are present.
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Study
these properties and relate them to
the bonding in each solid.
Francine
Taylor-Campbell teaches at Jamaica
College. Send questions and comments
to kerry-ann.hepburn@gleanerjm.com
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