离子化合物的晶格能是指在标准条件下，它的成分离子（阴离子和阳离子）在气态时结合成一摩尔离子晶体时所放出的能量。离子半径越大，晶格能越小。而离子的电荷越大，晶格能就越大。晶格能通常不能直接测出，但可通过玻恩-哈伯循环计算出。

The lattice energy of a crystalline solid is usually defined as the energy of formation of the crystal from infinitely-separated ions and as such is invariably negative. The concept of lattice energy was originally developed for rocksalt-structured and sphalerite-structured compounds like NaCl and ZnS, where the ions occupy high-symmetry crystal lattice sites. In the case of NaCl, the lattice energy is the energy released by the reaction