Concept: Lattice energy is the energy required to completely separate one mole of a solid ionic compound into gaseous ionic constituents. It is a measure of the strength of the electrostatic forces between ions in a crystal lattice.

Why (D) is correct: The lattice energy ($U$) of an ionic compound is directly proportional to the product of the charges on the ions ($q_1$ and $q_2$) and inversely proportional to the distance between their centers (which is related to the sum of their ionic radii, $r_1+r_2$). This relationship is described by the Born-Landé equation or qualitatively by Coulomb's Law:

$U\propto \frac{|q_1{q}_2|}{r_1+r_2}$

Therefore, both the magnitude of the charges on the ions and their sizes (which determine the interionic distance) significantly influence the lattice energy.

Option Analysis:

• A) Charge on the ions only: Incorrect. While charge is a major factor, the size of the ions also plays a crucial role.
• B) Size of the ions only: Incorrect. While size is a major factor, the charge on the ions also plays a crucial role.
• C) Packing of the ions only: Incorrect. While packing arrangement (crystal structure) can have a minor influence, the primary determinants are charge and size, which dictate the strength of individual electrostatic interactions.
• D) Charge and size of the ions: Correct. Higher charges lead to stronger electrostatic attraction and thus higher lattice energy. Smaller ionic radii lead to closer proximity of ions, resulting in stronger electrostatic attraction and higher lattice energy.

Correct Answer: (D)