Properties of Ionic Compounds
Revised by Dr. Walt Volland March 28, 2005 all rights reserved copyright 1999-2005
Ionic
solids are rigid because the multiple interactions between
oppositely charged ions hold the charged particles in relatively
fixed positions. Electrical conductivity depends on ions being
able to move freely. The conduction of heat depends partly
on the movement of electrons in a substance. Electric charge
is "locked" in the lattice positions of the ions in the solids.
This means there is poor mobility of charges and poor conductivity
of electricity and heat. Ionic
solids are brittle and hard because the electrostatic attractions
in the solid again hold the ions in definite positions. The
electrostatic attractions must be overcome to move the ions.
When the ions in the solid are shifted by some very strong
force the positions of ions shift so that like charged ions
are close together. This results in strong repulsions and
the like charged ions move apart. This causes the solid to
shatter and not simply deform like a metal. Try crushing a
few grains of salt in the bowl of a spoon with anoterh spoon.
The particles do not deform, they shatter. Iionic
solids melt when the ions have enough energy to slide past
one another. They are mobile and can act to carry electrical
charge through the liquid. This explains why a molten ionic
substance conducts electricity, but a solid ionic material
doesn't. The ions move through the liquid to carry charge
from one place to another. The
dissolving process is like a tug of war. Soluble ionic substances
are ripped apart by the solvent when the solid is dissolved.
The solvent pulls the ions out of the solid and breaks the
forces holding the crystal together. The solvent isolates
the ions in an envelope of solvent particles. The ions are
free to move and carry electric charge through the solution.
Nonsoluble
ionic solids are held together so tightly that the ions cannot
be pulled out of the lattice by the solvent. The attractive
forces in the solid are stronger than the attractions between
the solvent and the ions. The solvent can't pull the ions
out of the crystal.
Ionic compound
name Melting
point in degrees celsius LiCl lithium chloride 613 45 g cold water,
128 g100 NaCl sodium chloride 801 209 g cold water,
284 g 100 KCl potassium chloride 776 35 g cold water,
57 g 100 RbCl rubidium chloride 715 77 g cold water,
139 g100 MgF2 magnesium fluoride 1396 0.0076 g cold water,
insoluble 100 MgCl2 magnesium chloride 708 54 g cold water,
72 g100 MgBr2 magnesium bromide 695-700 101 g cold water,
126 g 100 MgI2 magnesium iodide greater than 700 100 g 0 cold
water, 165 g 100
Revised by Dr. Walt
Volland March 28, 2005 all rights reserved copyright 1999-2005
