5. Gain and Loss of Electrons   Previous PageNext Page
       Ions and Salts

Molten LiF is a rather unusual liquid compared to benzene or water. The interactions between particles are not the weak van der Waals forces of benzene, or the moderately strong dipole attractions and hydrogen bonds of water, but are the extremely strong electrostatic attractions between ions of opposite charge and the equally strong repulsions between ions of the same charge. Ions can move past one another in the fluid, but there is a tendency for each kind of ion to surround itself with ions of the opposite charge.

If we cool molten LiF to 842°C, the ions move so slowly that they lock into a regular array, with positive and negative charges alternating so that ions of the same charge are shielded from one another. This is the structure of LiF salt crystals, introduced in Chapter 4 and shown in the margin of the preceding page.

The individual and ions remain even in the solid, in marked contrast to the behavior of HF. The key to this new behavior is the difference in electronegativities of the atoms.

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