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

Atoms or ions in any solid do not sit rigidly in place. Their thermal energy causes them to vibrate about fixed positions in the crystal structure. The higher the temperature, the more energy the atoms in a solid have, and the more they vibrate.

The melting point of LiF salt crystals, 842°C, is the temperature at which the ions have enough vibrational energy to shake loose from the crystal structure and circulate past one another in a liquid.

The boiling point of molten LiF, 1676°C, is the temperature at which the particles in the melt can break loose and enter the gas phase. To do so they must pair off into Li-F molecules again. An isolated ion of one charge is tolerable when it is surrounded by ions of the opposite charge in a crystal, but isolated gaseous ions of and would require much greater energies. This charge separation is avoided by the formation of LiF diatomic molecules.


To produce such a LiF molecule, even though the bond is quite polar, the must give some of the borrowed electron pair back to . The high boiling point of the salt demonstrates that this is a hard thing to do. The ion does not really want the electron, and accepts it only at high temperatures (high energies).

In contrast to liquid LiF (a molten, ionic salt), liquid HF already exists as molecules, so all that is involved in its vaporization from liquid to gas is overcoming van der Waals forces and hydrogen bonds. This can be done at temperatures as low as 19.5°C.

  Page 07 of 57 HomeGlossary