Lattice energy is the energy required to convert one mole of crystal into its constituent ions in the gas phase at infinite separation. Ionic energy can also be defined to the energy released when a ions form an ionic solid.
M+(g) + X-(g) --> MX(s)
where M is a cation and X is an anion
Two trends when observing lattice energies:
1. The larger the ionic radii of the ions (cation or anion), the farther the electrons are from the nucleus, the easier it is to transfer electrons to form a solid, thus, less energy is required.
This can also be explained by Coloumb's Law
Therefore, lattice energy decreases from right to left and towards the bottom of the periodic table.
2. The divalent ions require a larger lattice energy to form a solid than monovalent ions. This is because twice as much energy is required to remove the second electron from a cation.