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 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.

The first step is to analyze each statement separately and consider its relevance to the stability of CaO. (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.

Now let's discuss the statements one by one. (a) The lattice energy is the energy required to separate one mole of a solid ionic compound into its gaseous ions. This energy contributes to the overall stability of ionic compounds. In the case of CaO, it's an ionic compound with Ca having a +2 charge and O having a -2 charge. This creates a strong attraction between the ions, leading to a large lattice energy. The larger the lattice energy, the more stable the compound. Therefore, this statement is a possible valid explanation. (b) CaO is an ionic compound, not a covalent one. This statement is not relevant to the discussion of CaO's stability and can be ruled out. (c) The molar mass of a compound has no direct influence on its stability. Therefore, this statement is not a valid explanation. (d) The enthalpy of formation measures the energy change when a compound is formed from its constituent elements. A smaller enthalpy of formation indicates that less energy is required to form the compound, but it doesn't explain CaO's stability compared to its elements. This statement is not the best explanation. (e) The stability of CaO to atmospheric conditions is irrelevant to the discussion of the compound's formation and energy processes. This statement is not a valid explanation.

Comparing the given statements, the best explanation for CaO's stability relative to its constituent elements is: (a) The lattice energy of CaO is large enough to overcome these processes. This is because a large lattice energy indicates strong attractive forces between the ions in the solid-state. This energy contributes to the overall stability of the compound, making it favorable despite the energy required for electron transfer processes.