Question

In an ionic compound, the size of the ions affects the internuclear distance (the distance between the centers of adjacent ions), which affects lattice energy (a measure of the attractive force holding those ions together). For the following ionic salts, KF, KCI, KBr, and KI: Based on ion sizes, a. rank these compounds by the size of the ion, and b. rank these ionic compounds by the size of their expected lattice energy

Answer 1

a) K⁺ is common is all, the only difference is anions, all are same group anions and down the group, size increases, the order of the size of anions will be: F^- < Cl^- < Br^- < I^-
hence, overall size order will be: KF < KCl < KBr < KI (increasing order of size)
b) Lattice energy depends on the inter-ionic distance between cations and anions, smaller the distance between them, stronger will be the force of attraction, hence higher will be lattice energy of the compound.
as F is smallest while I is largest,
therefore, the distance between cation and anion will be smallest in KF and largest in KI,
thus, increasing order of lattice energy will be: KI < KBr < KCl < KF