Ionic substances form 3-d arrays of alternating cations and anions that charge balance each other. This one is called face-centered cubic.

To calculate the lattice energy we must think of all the steps necessary to make a crystal from it's elements. This is the amount of energy given off when 1 mole of the solid is produced.

We will use Hess's Law to rearrange the steps so that we get the net equation we are after. We will sum the energies of each step and that will be our total lattice energy. Remember if you flip an equation, you must reverse its sign. If you multiply an equation remember to multiply the corresponding energy.

Example: Calculate the lattice energy of LiBr given the following

Atomization energy Br2  111.7 kJ/mol
Ionization energy Li 520.0 kJ/mol
Vaporization energy Br 15.46 kJ/mol
Vaporization energy Li 134.7 kJ/mol
Ionize Br2 -324.0 kJ/mol
Form LiBr -351.2 kJ/mol

Write an equation for each energy step. I like to order the steps in a possible sequence.

Vaporize Li Li (s) --> Li (g)  134.7 kJ/mol
Vaporize Br2 Br2(l) --> Br2(g) 15.46 kJ/mol
Atomize Br2 Br2 --> 2Br 111.7 kJ/mol
Ionize Li Li --> Li+ +e- 520.0 kJ/mol
Ionize Br Br +e- --> Br-

-324.0 kJ/mol
Form LiBr Li(s) + 1/2 Br2(l) --> LiBr(s) -351.2 kJ/mol

Now we re-arrange these steps so that we get a net equation for:

 Li+ + Br- --> LiBr(s)

flip Vaporize Li Li(g) --> Li (s)  -134.7 kJ/mol
flip Vaporize Br2 (1/2) Br2(g) --> Br2(l) (1/2) -15.46 kJ/mol
flip Atomize Br2 (1/2) 2Br --> Br2 (1/2) -111.7 kJ/mol
flip Ionize Li Li+ +e- --> Li -520.0 kJ/mol
Ionize Br Br +e- --> Br-

-324.0 kJ/mol
Form LiBr Li(s) + 1/2 Br2(l) --> LiBr(s) -351.2 kJ/mol

Check cancellation to get desired net equation

 Li+ + Br- --> LiBr(s)

 Vaporize Li Li(g) --> Li (s)  -134.7 kJ/mol
 Vaporize Br2 (1/2) Br2(g) --> Br2(l) (1/2) -15.46 kJ/mol
 Atomize Br2 (1/2) 2Br(g) --> Br2(g) (1/2) -111.7 kJ/mol
 Ionize Li Li+ +e- --> Li(g) -520.0 kJ/mol
 Ionize Br Br- --> Br(g) +e-

-324.0 kJ/mol
 Form LiBr Li(s) + 1/2 Br2(l) --> LiBr(s) -351.2 kJ/mol
Lattice energy   -1393.48 kJ/mole

 Li+ + Br- --> LiBr(s)