Question about equation for lattice energy?
For a compound like Mg(OH)2, the q for hydroxide would be doubled because there are two of them.
Lattice energy in a ionic compound is proportional to the energy expended in the production of the compound. As the compound becomes more complex by the addition of more ions to the crystal lattice structure, more energy is needed.
Four steps involved in forming an element into a crystal consist of:
1) changing a solid (metal) into its gaseous state
2) changing the gaseous solid into an ion
3) changing a diatomic gas into is elementary form (if necessary)
4) combining the ions into a crystalline structure
The mathematical discussion of this situation is located here if you want to see the process - it touches on your question starting at 5:30 onwards:
Another good (short) discussion on Lattice energy and its affect on dissolving compounds is located here: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch7/lattice.html