8.2. Ionic Bonds and Ionic Compounds http://www.ck12.org
Ionic Compounds
The electron dot diagrams show the nature of the electron transfer that takes place between metal and nonmetal
atoms. However, ionic compounds do not exist as discrete molecules, as the dot diagrams may suggest. In
order to minimize the potential energy of the system, as nature prefers, ionic compounds take on the form of an
extended three-dimensional array of alternating cations and anions. This maximizes the attractive forces between
the oppositely charges ions. The figure below (Figure8.4) shows two different ways of representing the ionic
crystal lattice. A ball and stick model makes it easier to see how individual ions are oriented with respect to one
another. A space filling diagram is a more accurate representation of how the ions pack together in the crystal.
FIGURE 8.4
Two models of a sodium chloride crystal
are shown. The purple spheres represent
the Na+ions, while the green spheres
represent the Cl−ions. (A) In an ex-
panded view, the distances between ions
are exaggerated, more easily showing the
coordination numbers of each ion. (B) In
a space filling model, the electron clouds
of the ions are in contact with each other.Coordination Number
Thecoordination numberis the number of ions that immediately surround an ion of the opposite charge within
a crystal lattice. If you examine the figure above (Figure8.4) (A), you will see that there are six chloride ions
immediately surrounding a single sodium ion, so the coordination number of sodium is 6. Likewise, six sodium ions
immediately surround each chloride ion, making the coordination number of chloride also equal to 6. Because the
formula unit of sodium chloride displays a 1:1 ratio between the ions, the coordination numbers must be the same.
The formula unit for cesium chloride is CsCl, also a 1:1 ratio. However, as shown below (Figure8.5), the
coordination numbers are not 6, like they are in NaCl. The center Cs+ion is surrounded by eight Cl−ions at
the corners of the cube. Each Cl−ion is also surrounded by eight Cs+ions. The coordination numbers in this type
of crystal are both 8. CsCl and NaCl do not adopt identical crystal packing arrangements because the Cs+ion is
considerably larger than the Na+ion.
Another type of crystal is illustrated by titanium(IV) oxide, TiO 2 , which is commonly known as rutile. The rutile
crystal is shown below (Figure8.6).
The gray Ti^4 +ions are each surrounded by six red O^2 −ions. The O^2 −ions are each surrounded by three Ti^4 +ions.
The coordination of the titanium(IV) cation is 6, which is twice the coordination number of the oxide anion, which
is 3. This fits with the formula unit of TiO 2 , since there are twice as many O^2 −ions as Ti^4 +ions.
The crystal structure of any ionic compound must reflect its formula unit. For example, in a crystal of iron(III)
chloride, FeCl 3 , there are three times as many chloride ions as iron(III) ions.
View an animated example at http://www.dlt.ncssm.edu/core/Chapter9-Bonding_and_Geometry/Chapter9-Animati
ons/IonicBonding.html
- How does this animation represent the transfer of electrons?