Chemistry Reference
In-Depth Information
molecular substances have higher melting points,
because the whole structure is held together by
strong covalent bonds.
Giant molecular
or
macromolecular
structures
contain many hundreds of thousands of atoms joined
by strong covalent bonds. Examples of substances
showing this type of structure are diamond, graphite,
silicon(
iv
) oxide (Figure 3.31) and plastics such as
polythene (Chapter 14, p. 226). Plastics are a tangled
mass of very long molecules in which the atoms are
joined together by strong covalent bonds to form
long chains. Molten plastics can be made into fibres
by being forced through hundreds of tiny holes in a
'spinneret' (Figure 15.19, p. 241). This process aligns
the long chains of atoms along the length of the fibre.
For a further discussion of plastics, see Chapter 14,
pp. 226-9 and Chapter 15, p. 241.
It should be noted that in ionic compounds the
interionic forces are much stronger than the
intermolecular forces in simple covalent substances and
so the melting and boiling points are generally higher.
●
Generally, they do not conduct electricity when
molten or dissolved in water. This is because they
do not contain ions. However, some molecules
actually react with water to form ions. For example,
hydrogen chloride gas produces aqueous hydrogen
ions and chloride ions when it dissolves in water:
HCl(
g
)
water
H
+
(
aq
) + Cl
−
(
aq
)
●
Generally, they do not dissolve in water. However,
water is an excellent solvent and can interact with and
dissolve some covalent molecules better than others.
Covalent substances are generally soluble in organic
solvents. For a further discussion of solubility of
substances in organic solvents see Chapters 14 and 15.
O
Si
Allotropy
When an element can exist in more than one physical
form in the same state it is said to exhibit
allotropy
(or polymorphism). Each of the different physical
forms is called an
allotrope
. Allotropy is actually
quite a common feature of the elements the Periodic
Table (p. 136). Some examples of elements which
show allotropy are sulfur, tin, iron and carbon.
Allotropes of carbon
Carbon is a non-metallic element which exists in
more than one solid structural form. Its allotropes are
called
graphite
and
diamond
. Each of the allotropes
has a different structure (Figures 3.32 and 3.33) and
so the allotropes exhibit different physical properties
(Table 3.7). The different physical properties that
they exhibit lead to the allotropes being used in
different ways (Table 3.8 and Figure 3.34).
Table 3.7
Physical properties of graphite and diamond.
a
The silicon(
iv
) oxide structure in quartz.
b
Quartz is a hard solid at room temperature. It has a melting point of
1610 °C and a boiling point of 2230 °C.
Figure 3.31
Properties of covalent compounds
Covalent compounds have the following properties.
●
As simple molecular substances, they are usually
gases, liquids or solids with low melting and boiling
points. The melting points are low because of the
weak intermolecular forces of attraction which
exist between simple molecules. These are weaker
compared to the strong covalent bonds. Giant
Property
Graphite
Diamond
Appearance
A dark grey, shiny
solid
A colourless transparent
crystal which sparkles in light
Electrical
conductivity
Conducts electricity
Does not conduct electricity
Hardness
A soft material with
a slippery feel
A very hard substance
Density/g cm
−
3
2.25
3.51
