Ionic Bonds and Lattice Energy Conceptually, lattice energy is the energy of an ionic bond. Lattice energy is defined experimentally as the energy needed to break apart a solid ionic crystal into its individual gaseous ions. Example: NaCl (s) ® Na + (g) + Cl – (g) The magnitude of lattice energies can be compared qualitatively on the basis of two variables (based on Coulomb’s law: 1. The charge difference between the two ions. This factor is the most important of the two. The greater the charge difference, the greater the lattice energy. 2. The size of the two ions since this determines the distance between the two ions. The smaller the ions, the greater the lattice energy. Example: Which ionic compound would have the larger lattice energy and therefore the stronger ionic bond: Magnesium oxide or sodium fluoride. Explain your answer. Since charge difference is the most important factor, consider this factor first. Magnesium oxide consists of Mg 2+ and O 2- ions so the charge difference is 4. Sodium fluoride consists of Na + ions and F – ions with a charge difference of 2. The charge difference in magensium oxide is larger so this compound will have the higher lattice energy and the stronger ionic bond. Example: Which ionic compound would have the larger lattice energy and therefore the stronger ionic bond: sodium chloride or cesium chloride. Explain your answer. Since charge difference is the same for both compounds, the lattice energy will be determined by the size of the ions. Cs + has the 5th energy level as its outer energy level while Na + has the 2nd energy level as its outer energy level. The Cs + ion is larger and therefore the distance between the positive and negative ions in cesium chloride is greater. A greater distance results in a weaker lattice energy for cesium chloride.
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