The Ionic Bond
When a highly electronegative atom and an electropositive one are
bonded together, an
electron is transferred from the electropositive atom to the
electronegative atom to form a cation
and an anion, respectively. The cation, being a positively charged
ion, is attracted to the
negatively charged anion as described by Coulomb's law:
Figure 1.1: Coulomb's law states that oppositely charged species attract each
other.
A negative energy means there is an attractive interaction between the particles
in the . If the charges on the two ions are opposite in sign, they
will attract each
other. Conversely, if two
charges are similar, they repel each other. Using this knowledge we can
construct a graph of energy
versus distance for two oppositely charges ions. At large distances, there
is a negligible energy of
attraction between the two ions, but as they are brought closer together,
they are attracted to one another. Coulomb's law may seem to predict that the
ions should be as close
as possible to achieve a
minimal energy state. However, the shows that the
ions are actually repelled at small distances. To explain this
observation, remember that the ions'
nuclei are both positively charged. When the nuclei approach each other,
they repel strongly--accounting for the steep rise in potential as the ions get
closer
than the bond length.
Figure 1.2: Plot of potential energy versus distance for oppositely charged ions
The depth (y-axis) of the minimum in the potential energy curve above represents
the
bond strength, and the distance (x-axis) at the energy minimum is the bond
length. Using Coulomb's
law and the bond length, one can actually predict with some accuracy the
strength of an ionic bond.
Performing a series of these calculations you find that ionic compounds
formed by ions with larger
charges create stronger bonds and that ionic compounds with shorter bond
lengths form stronger
bonds.
Crystal Lattices
Ionic compounds do not usually exist as isolated molecules, such as LiCl,
but as a part of a crystal
lattice--a three dimensional regular array of cations and anions.
Ionic compounds form lattices
due to the contributing coulombic attractions of having each cation surrounded
by several anions and
each anion surrounded by several anions. An example of a crystal lattice
is shown in :
Figure 1.3: An ionic crystal lattice
As you can see in the above figure, each lithium ion is surrounded by six
chlorine atoms and vice
versa. By virtue of the arrangement of the ions in the lattice, the
lattice is lower in energy than it would be if the
ions were separated into isolated LiCl molecules.
