Question

The internuclear distances in gas-phase ionic molecules are considerably smaller than those in the corresponding crystals. For example, the internuclear distance in NaCl(g) is 2.3606 Å, whereas in NaCl(s) the shortest Na-Cl distance is 2.814 Å. Explain why this should be so when the binding energy is larger in the crystal.

Answer 1

The internuclear distances in gas-phase ionic molecules are considerably smaller than those in the corresponding crystals. This is due to difference in intermolecular forces and structure of crystal. In gas phase intermolecular forces are very weak that means molecules are almost isolated molecules, so there is no conderable attraction between adjacent molecules. But in crystals they atoms are in particular arrangement the attraction between Chloride ion in one molecule to sodium ion in another molecule is more effective. Because molecules in solid state are very closer.

Hence the internuclear diastance in crystals is greater tha internuclear distance in gas phase.