The lattice energy of ionic compounds is influenced by the size and charge of the constituent ions.

Lattice energy refers to the amount of energy required to separate one mole of an ionic compound into its gaseous ions. This energy is a key indicator of the strength of the ionic bonds that hold the ions together in the crystal lattice. Specifically, lattice energy is affected by both the size and the charge of the ions involved: smaller ions and higher charges lead to stronger ionic bonds and, consequently, higher lattice energies.

For instance, the lattice energy of sodium chloride (NaCl) is $-787 \, \text{kJ/mol}$. This relatively moderate lattice energy can be attributed to the sodium ion (Na$^+$), which is small and has a charge of $+1$, and the chloride ion (Cl$^-$), which is also small and carries a charge of $-1$. The strong electrostatic attraction between these oppositely charged ions contributes to the significant lattice energy.

In comparison, the lattice energy of calcium oxide (CaO) is $-3,330 \, \text{kJ/mol}$. Here, the calcium ion (Ca$^{2+}$) is larger than the sodium ion and has a charge of $+2$, while the oxide ion (O$^{2-}$) is larger than the chloride ion and possesses a charge of $-2$. The combination of larger ionic sizes and higher charges results in a more substantial electrostatic attraction, leading to a much higher lattice energy.

In summary, the lattice energy of ionic compounds is a crucial factor that influences their physical and chemical properties, such as melting and boiling points, solubility, and reactivity. Understanding lattice energy helps predict how these compounds will behave under various conditions.