http://www.ck12.org Chapter 8. Ionic and Metallic Bonding
FIGURE 8.5
In a cesium chloride crystal, the cesium ion (orange) occupies the center,
while the chloride ions (green) occupy each corner of the cube. The
coordination number for both ions is eight.FIGURE 8.6
Titanium(IV) oxide forms tetragonal crys-
tals. The coordination number of the Ti^4 +
ions (gray) is 6, while the coordination
number of the O^2 −ions (red) is 3.- How do the sodium chloride units join together?
To better understand the structures and properties of ionic compounds, go to http://www.chemguide.co.uk/atoms/str
uctures/ionicstruct.html.
Physical Properties of Ionic Compounds
Pictured below (Figure8.7) are a few examples of the color and brilliance of naturally occurring ionic crystals.
The regular and orderly arrangement of ions in the crystal lattice is responsible for the various shapes of these
crystals, while transition metal ions give rise to the colors.
Because of the many simultaneous attractions between cations and anions that occur, ionic crystal lattices are very
strong. The process of melting an ionic compound requires the addition of large amounts of energy in order to break
all of the ionic bonds in the crystal. For example, sodium chloride has a melting temperature of about 800°C.
Ionic compounds are generally hard but brittle. Why? It takes a large amount of mechanical force, such as striking
a crystal with a hammer, to force one layer of ions to shift relative to its neighbor. However, when that happens,
it brings ions of the same charge next to each other (Figure8.8). The repulsive forces between ions of the same
charge causes the crystal to shatter. When an ionic crystal breaks, it tends to do so along smooth planes because of
the regular arrangement of the ions.