What Is Lattice Energy?

Andrew Kirmayer

Lattice energy is a form of atomic potential energy. It is the amount of energy needed to split up the ions of a solid into gaseous ions and atoms, from which a positive value is derived. The reverse form, in which energy is released as ions bind during the formation of a solid, yields a negative value. The standard units for lattice energy values are kilojoules per mols (kJ/mol), and the primary application for calculating this atomic energy is to describe how stable a particular ionic solid is.

Scientist with beakers
Scientist with beakers

Ionic solids have a crystalline structure, so each ion can interact with a few others of the opposite charge. The ordered structure allows ions to interact despite the available thermodynamic energy to drive any process, or entropy, being low. When ions of opposite charges do interact, a great deal of energy is released, a property that causes ion solids to have very high melting and boiling points.

The energy needed for these reactions can be determined by calculating the lattice energy for the ions. Lattice energies are calculated by applying the Born-Haber Cycle, which involves a combination of energy concepts used to derive an accurate measurement. Ionization energy, the energy needed to get an electron to separate from an atom or ion, as well as electron affinity, the energy released by adding an electron to an ion or neutral atom, are included in such calculations. Other values include dissociation energy, need to break apart a compound; sublimation energy, which can cause a substance to change from a solid to gas; and heat of formation, the change in energy when a compound is created out of its basic elements.

Changes in energy in any process can be calculated by breaking things down step-by-step, which is explained by Hess’ Law. By reorganizing the Born-Haber Cycle equation to calculate lattice energy, the result of subtracting the heat of formation from the heat of atomization, from the dissociation energy value, minus the sum of ionization energies, minus the sum of electron affinities is attained. This result describes the release of energy when metal and nonmetal ions form a solid. Lattice energy trends define whether the solid is being broken up, for which the end result is a positive value, or ions are binding, resulting in a negative energy value. These energy values are always estimated using Born-Haber Cycle principles because lattice energy cannot be directly measured.

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