E The Ionic Bond

In the formation not of a molecule but of a crystal of sodium chloride still more energy is set free. In a crystal of the type of 

For a crystal, built up of molecules, the lattice energy is but small, as evidenced by the volatility, for example, of most organic substances (see however p. 160 about diamond etc.) in contrast to the hardly volatile salt-like compounds, which crystallize in an ionic lattice. 

A molecule consisting of oppositely charged ions attracting each other, and still less a crystal constructed of ions, could not 

It is not so simple to prove directly that rock-salt, for example, is constructed from ions. The fact that the melt and the aqueous solution, as appears from the conduction of the electric current and electrolysis, consist of ions, forms no conclusive- 

According to Dulong and Petit s rule the specific heat per gramatom amounts to about 6 cal./deg. Since now the specific heat per grammol. is 11.9 cal./deg. for NaCl at o° C, it follows that there must be two particles per molecule of NaCl. The specific heat depends, however, on the temperature, and deviations from this rule, both upwards and downwards, are numerous so that the argument is not so significant. For LiCl, LiH and MgO at o° C one finds 9.7, 7.8 and 8.4 cal./deg., respectively, for the molecular heat nevertheless these are also ionic lattices. 

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