Energy is required to remove two electrons from Ca to form \(\mathrm{Ca}^{2+},\) and energy is required to add two electrons to \(\mathrm{O}\) to form \(\mathrm{O}^{2-}\). Yet \(\mathrm{CaO}\) is stable relative to the free elements. Which statement is the best explanation? (a) The lattice energy of \(\mathrm{CaO}\) is large enough to overcome these processes. (b) \(\mathrm{CaO}\) 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 \(\mathrm{CaO}\) is small. (e) \(\mathrm{CaO}\) is stable to atmospheric conditions.

CaO (calcium oxide) is an ionic compound formed by the transfer of electrons from a metal atom (calcium) to a non-metal atom (oxygen). In this process, two electrons are removed from calcium (Ca) to form Ca²⁺ ion, and two electrons are added to oxygen (O) to form O²⁻ ion.

Let us go through the given statements one by one and analyze their relevance to the stability of CaO. (a) The lattice energy of CaO is large enough to overcome these processes. Lattice energy is the energy required to break a solid ionic compound into gaseous ions. A more significant lattice energy value means a more stable ionic compound. Thus, this statement could be a reason for the stability of CaO. (b) CaO is a covalent compound, and these processes are irrelevant. This statement is incorrect because CaO is an ionic compound, not a covalent one. (c) CaO has a higher molar mass than either Ca or O. The molar mass of a compound is not responsible for the stability of the compound, so this statement can be ignored. (d) The enthalpy of formation of CaO is small. A negative enthalpy of formation indicates an exothermic reaction and might contribute to the stability of the compound. However, it is not the only factor determining the stability of CaO. (e) CaO is stable to atmospheric conditions. Although this is a correct statement, it doesn't explain the inherent stability of CaO relative to the free elements.

Based on our analysis, the best explanation for the stability of CaO relative to the free elements is (a) The lattice energy of CaO is large enough to overcome these processes. The high lattice energy compensates for the energy required to remove electrons from Ca and add electrons to O, resulting in a stable ionic compound.