Energy is required to remove two electrons from Ca to form Ca2+, and energy is required to add two electrons to O to form O2 - . Yet CaO is stable relative to the free elements. Which statement is the best explanation? (a) The lattice energy of CaO is large enough to overcome these processes. (b) CaO is a covalent compound, and these processes are irrelevant. (c) CaO has a higher molar mass than either Ca or O. (d) The enthalpy of formation of CaO is small. (e) CaO is stable to atmospheric conditions.

Key Concepts

Lattice Energy

Lattice energy is the energy released when gaseous ions combine to form an ionic solid. It is a crucial factor in determining the stability of ionic compounds like CaO. A large lattice energy indicates that the electrostatic forces between the ions are strong enough to compensate for the energy required to remove or add electrons, thus stabilizing the compound.

Ionization Energy and Electron Affinity

Ionization energy is the energy required to remove an electron from an atom, while electron affinity is the energy change when an electron is added to an atom. In the case of Ca and O, the energy needed to ionize Ca and add electrons to O must be considered when evaluating the overall stability of the resulting ionic compound, CaO.

Enthalpy of Formation

The enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. A small enthalpy of formation for CaO suggests that the formation process is energetically favorable, contributing to the stability of the compound despite the energy costs associated with ionization and electron addition.