Lattice Energy
Definition
The energy released when separated gas-phase ions come together to form a solid crystal. Bigger lattice energy means a more stable, higher-melting salt.
Symbol: · Unit: kJ/mol
Technical definition
Lattice energy is the energy released when one mole of a solid ionic compound forms from its gaseous ions. It increases with ion charge and decreases with ion size, correlating with melting point, hardness, and solubility.
Example
MgO has a lattice energy of about 3795 kJ/mol, far larger than NaCl's 787 kJ/mol, because Mg^2+ and O^2- carry double charges. That is why MgO melts at 2852 C.
Lattice energy is defined for the formation of the solid from gaseous ions, which is exothermic. Some textbooks quote the reverse process, separating the lattice, which is endothermic, so check the sign convention.