|
Structure
and bonding Pt. III
Francine
Taylor Campbell, Contributor
 |
| These
students of the Stony Hill HEART
Academy were seen looking through
a book before heading to their
classes. - Carlington Wilmot Photo |
IN
THIS week's lesson we will be comparing
the properties of ionic solids and giant
molecular crystals such as diamond and
graphite.
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
diamond 4 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
Hardness |
Crystalline
solid
Brittle
easily split
|
Sparkling
solid
Very
hard due
to
strong covalent bonds in the
structure.
|
Dark
solid
Soft
and flaky
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 coalent bonds
that need vast amounts of energy
to break. |
Very
high due to strong coalent 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.
|
