Lattice energy is a measure of just how much stabilization results from arranging oppositely charged ions in an ionic compound.
Lattice energy is the energy required to completely separate a mole of solid ionic compound into its gaseous ions.
Lattice Energy can actually be described in 2 ways: one way is the energy released when gaseous ions combine to form an ionic solid. The other is the energy required to break an ionic solid into its component gaseous ions. Either way, we can see that lattice energy is a measure of the forces between the ions in an ionic solid.
The more stable the ionic solid, the larger the lattice energy will be. This is because the more stable bonds are, the harder they are to break.
More Lattice energy = more stable = harder to break = stronger bond
So how do we know how much lattice energy an ionic bond has?
Ions with greater charge have greater lattice energy.
For example, sodium fluoride has a lattice energy of 923 kJ/mol while the lattice energy of another ionic solid, magnesium fluoride, is 2957 kJ/mol. Here, magnesium has a charge of +2, while sodium's charge is +1. The greater charge present in magnesium fluoride holds the ions together more tightly, raising the lattice energy.
Compounds that have smaller ions will have higher lattice energies.
Ionic radius is a measure of the size of an ion. Compounds that have smaller ions will have higher lattice energies because the ions can get closer together. So remember the trend for atomic radius -- you may need to be able to tell the difference in size between two atoms.
When elements come together to form a bond, we can tell how much "ionic character" that bond has by looking at the electronegativities of the ions that it is made of.
The bigger the difference in electronegativities, the higher the ionic character.
So, remember the trend for electronegativity -- Fluorine has the highest -- increases from left to right, decreases as you go down.
So if you had a compound made of K and Cl, it would have a very high ionic character (because K has a very low electronegativity and Cl has a high electronegativity -- big difference in electronegativies means big ionic character)
If you had O and Cl, it would have a very low ionic character (because O and Cl both have high electronegativies, that are very close to one another -- small difference in electronegativy, small ionic character)