Final answer:
The lattice energies of alkali-metal fluorides decrease as the size of the alkali-metal ions increases, which leads to an increase in the distance between the metal cations and the fluoride anions, reducing the electrostatic attraction between them. Option number C is correct.
Step-by-step explanation:
The trend in the lattice energies of the alkali-metal fluorides such as LiF, NaF, KF, and RbF shows a decrease as we move down the alkali metal group in the periodic table. This trend is due to the increasing size of the alkali-metal ions from lithium to rubidium, which leads to an increase in the interionic distances between the metal cations and the fluoride anions.
As the distance between ions increases, the electrostatic attraction between them decreases, resulting in lower lattice energy.
For example, LiF, with a smaller lithium ion, has a higher lattice energy compared to NaF, KF, or RbF. The larger the ion, the farther apart the ions are in the ionic lattice, and thus the weaker the forces holding them together. This is consistent with Coulomb's Law, which states that the force between two charges is inversely proportional to the square of the distance between them.