bei48482_FM

268 Chapter Eight

rock salt, NaCl, where the bond exists between Naand Clions and not between

Na and Cl atoms (Fig. 8.2). Ionic bonds usually do not result in the formation of mol-

ecules. The crystals of rock salt are aggregates of sodium and chlorine ions which, al-

though always arranged in a certain definite structure (Fig. 8.3), do not pair off into

molecules consisting of one Naion and one Clion. Rock salt crystals may have any

size and shape. There are always equal numbers of Naand Clions in rock salt, so

that the formula NaCl correctly represents its composition. Molten NaCl also consists

of Naand Clions: these ions form molecules rather than crystals only in the gaseous

state. Ionic bonding is further discussed in Chap. 10.

Figure 8.2An example of ionic bonding. Sodium and chlorine combine chemically by the transfer of

electrons from sodium atoms to chlorine atoms; the resulting ions attract each other electrically.

Na+

Cl–

Figure 8.3Scale model of an NaCl

crystal.

Cl

+17

Na

+11

Cl–

+17

+11

Na+

Figure 8.1(a) Orbit model of the hydrogen molecule. (b) Quantum-mechanical model of the hydro-

gen molecule. In both models the shared electrons spend more time on the average between the nuclei,

which leads to an attractive force. Such a bond is said to be covalent.

(a)

(b)

Electron

H

+=

H 2

Proton

High probability

of finding electrons

+=

HH H 2

Low probability

of finding electrons

H

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