Ionic Substances
Crystal lattice - regular repeating system in which a unit (for example Na+,
Cl-) structure of a particular shape appears again and again.
Crystal lattice energy - the energy liberated from one mole of ionic crystal
(one mole of solid ionic compound is completely separated into its gaseous
ions). This is an experimentally determined quantity that shows the stability
of the crystal.
Example:
Na(g) + Cl(g)  NaCl(s)
-761kJ
Bond Strength and Crystal Stability
When considering bond strength and crystal stability must take two factors
into account:
1. Charge
2. Radius
The radii of various ions are determined experimentally.
The force of attraction between the ions can be calculated quantitatively
using the following formula:
F = k q1q2
r2
where:
F - force of attraction
q1 & q2 - charge of ions
r - sum of radii of the cation and anion
Based on the above:
the larger the ionic radius, the ___________ the force of attraction
the smaller the ionic radius, the __________ the force of attraction
Note: when the charges of two ions being investigated are the same, then
the radius of the ions involved becomes crucial in determining the degree
of force of attraction.
KEY: Bond strength and stability is proportional to the force of attraction.
Thus the greater the force of attraction, the _________ the bond and the
greater the force of attraction the ___________ the stability.
Consider the following:
NaCl
bond:
lattice energy:
melting point:
solubility in cold water:
CsCl
bond:
lattice energy:
melting point:
solubility in cold water: