Final answer:
The compound with the lowest lattice energy, among the given options, is (B) KF (potassium fluoride), due to its monovalent ions and the larger ionic radius of potassium compared to the ions in the other compounds listed.
Step-by-step explanation:
The question asks which compound has the lowest (i.e., least negative) lattice energy. Lattice energy is the energy required to separate one mole of the solid ionic compound into its component gaseous ions. It is affected by the charge and size (or radius) of the ions that make up the compound. The higher the charge and the smaller the radius, the greater the lattice energy.
Among the given options:
- NaCl consists of Na+ and Cl- ions.
- KF consists of K+ and F- ions.
- MgO consists of Mg2+ and O2- ions.
- CaS consists of Ca2+ and S2- ions.
Considering the charges and radii of the ions in the compounds, the one with the lowest charge and largest ions would have the lowest lattice energy. This would typically be the compound with monovalent ions (i.e., ions with a charge of +1 or -1) and larger radii.
Based on this information, KF (potassium fluoride) would have a lower lattice energy than NaCl because K+ has a larger radius than Na+. Between the choices with divalent ions, MgO and CaS, CaS would have a lower lattice energy than MgO because Ca2+ has a larger radius than Mg2+.
Between KF and CaS, KF would have the lowest lattice energy because it has monovalent ions compared to the divalent ions in CaS. Therefore, the correct answer is (b) KF.