Chemistry Reference
In-Depth Information
Ionic structures
Ionic structures are solids at room temperature
and have high melting and boiling points. The
ions are packed together in a regular arrangement
called a
lattice
. Within the lattice, oppositely
charged ions attract one another strongly.
Figure 3.15d shows only a tiny part of a small
crystal of sodium chloride. Many millions of
sodium ions and chloride ions would be arranged
in this way in a crystal of sodium chloride to make
up the giant ionic structure. Each sodium ion in
the lattice is surrounded by six chloride ions, and
each chloride ion is surrounded by six sodium
ions.
Not all ionic substances form the same structures.
Caesium chloride (CsCl), for example, forms a
different structure due to the larger size of the
caesium ion compared with that of the sodium ion.
This gives rise to the structure shown in Figure 3.16,
which is called a body-centred cubic structure. Each
caesium ion is surrounded by eight chloride ions
and, in turn, each chloride ion is surrounded by
eight caesium ions.
Cs
CI
a
The arrangement of ions.
Cs
surrounded by 8 Cl
ions
In the same way, each Cl
is surrounded by 8 Cs
ions
b
The unit cell.
Figure 3.16
The structure of caesium chloride.
Properties of ionic compounds
Ionic compounds have the following properties.
●
They are usually solids at room temperature, with
high melting points. This is due to the strong
electrostatic forces holding the crystal lattice
together. A lot of energy is therefore needed to
separate the ions and melt the substance.
●
They are usually hard substances.
●
They usually cannot conduct electricity when solid,
because the ions are not free to move.
●
They mainly dissolve in water. This is because
water molecules are able to bond with both the
positive and the negative ions, which breaks up
the lattice and keeps the ions apart. Figure 3.17
shows the interaction between water molecules (the
solvent) and sodium and chloride ions from sodium
chloride (the solute). For a further discussion of
the solubility of substances see Chapter 8 (p. 129).
●
They usually conduct electricity when in the
molten state or in aqueous solution. The forces of
attraction between the ions are weakened and the
ions are free to move to the appropriate electrode.
This allows an electric current to be passed through
the molten compound (see Chapter 5, p. 73).
Na
+
Cl
-
O
H
H
Na
+
Cl
-
Figure 3.17
Salt (an ionic compound) dissolving in water.
