Chemistry, Third edition

4 · BONDING BETWEEN ATOMS

In the unit cell, each sodium ion is surrounded by six chloride ions and each chlo-

ride ion is surrounded by six sodium ions. The ions are held together by the attraction

of their opposite charges (electrostatic forces). These forces are very strong. X-ray

diffractionallows us to ‘see’ ions in an ionic compound, as described in Box 4.2.

All ionic substances consist of ions arranged in a giant lattice. Different ionic

structures have their ions arranged in different patterns. The sodium chloride struc-

ture is one of the simplest arrangements.

50

Cubic salt crystals. An orderly

arrangement of Naand Cl

ions gives rise to this shape.

BOX 4.2

Electron density map for sodium chloride

How do the ideas of Kossel and Lewis

fit in with the modern view of the

arrangement of electrons in an atom?

The modern theory teaches that

electrons have a wave-like nature. In

regions where there is a high probability

of finding the electron, there is a high

concentration of electrical charge and

we say that theelectron densityis

large. By similar reasoning, electron

density is small where there is a low

probability of finding the electron.

Through using a technique known as X-

ray diffraction, it is possible to obtain

electron density contour maps of ionic

substances. Figure 4.3 shows such a

map for sodium chloride – each line

connects points with the same electron

density, and the nuclei of each atom are

represented by dots at the centre of

each ion. The outermost contours have

a squared shape because the ions are

attracted to ions of opposite charge

around them. Notice that there is a

space between the nuclei where there

are no contours; this is a characteristic

feature of the electron density map for

an ionic substance.

Cl–

Na+

Fig. 4.3Contours to show the electron

density distribution around sodium and

chloride ions.

Fig. 4.2The unit cell of
sodium chloride: (a) ball-and-
stick model; (b) space-filling
model.

(a)

(b)

Arrangement of ions in sodium chloride

In a sodium chloride crystal, the Na+and Clions are arranged in a giant lattice

structure. The ‘building brick’ of this structure is a unit cellis as shown in Fig 4.2.

The larger spheres represent Clions, whereas the smaller spheres represent Na+

ions. A crystal of sodium chloride consists of many billions of these unit cells stacked

together in the lattice.