The Standard Lattice Enthalpy, DH°LE, 298, is the heat evolved when one mole of a crystalline ionic compound forms from its gaseous ions, measured under standard conditions.
Mg2+(g) + 2Br-(g) ® MgBr2(s) DH°LE, 298 = -2440kJ mol-1
The magnitude of the lattice enthalpy depends on how closely the ions pack together in the crystal structure. The more closely they pack the more exothermic the lattice enthalpy. The higher the charge on the ions and the smaller their size, the more closely they will pack in the crystal structure.
See also Hess's Law | Hydration Enthalpy | Enthalpy of Solution.