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The Solid State 339

element has a high electron affinity.Electron affinity is the energy released when an

electron is added to an atom of a given element; the greater the electron affinity, the

more such atoms tend to become negative ions. Sodium, with an ionization energy of

5.14 eV, tends to form Naions, and chlorine, with an electron affinity of 3.61 eV,

tends to form Clions. The condition for a stable crystal of NaCl is simply that the

total energy of a system of Naand Clions be less than the total energy of a system

of Na and Cl atoms.

The cohesive energyof an ionic crystal is the energy per ion needed to break the

crystal up into individual atoms. Part of the cohesive energy is the electric potential

energy Ucoulombof the ions. Let us consider a Naion in NaCl. From Fig. 10.5 its

nearest neighbors are six Clions, each one the distance raway. The potential energy

Cl–

0.562 nm Na+

Figure 10.5The face-centered cubic structure of

NaCl. The coordination number (the number of

nearest neighbors about each ion) is 6.

Electron micrograph of sodium chloride crystals. The cubic struc-

ture of the crystals is often disrupted by dislocations.

0.411 nm

Cs+

Cl–

Figure 10.6The body-centered cubic structure

of CsCl. The coordination number is 8.

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