|
Structure
and bonding III
Francine
Taylor-Campbell, Contributor
 |
| This
student uses a slab of concrete for his desk during class at the Seaforth High
School in St.Thomas. - Peta-Gaye Clachar/Staff Photographer |
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
POINTS
TO NOTE - 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
a 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 3 other atoms arranged hexagonally in layers.
These layers are held together by weak bonds which enables 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 the 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 a 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
electricity when solid as mobile electrons are present |
Francine
Taylor-Campbell is an independent contributor. | 
