Question

lattice energy is a measure of ionic - bond strength. the lattice energy/ionic bonding strength for lithium iodide (li⁺i⁻) is 757 kj/mol. the lithium cation (li⁺, 90 pm) is much smaller than the iodide anion (i⁻, 206 pm). the cesium cation (cs⁺, 181 pm) is similar in size to the iodide anion (220 pm). which of the following halide ions, when combined with cs⁺, would have a similar lattice energy/ionic bond strength as lithium iodide? fluoride (f⁻) chloride (cl⁻) bromide (br⁻) iodide (i⁻) dont test me yet, i am trying to gain an understanding before attempting to answer this question.

Explanation:

Step1: Recall lattice - energy factors

Lattice energy depends on ion - size and charge. Here, we focus on ion - size as the charge of the cations ($Cs^+$) and anions (halide ions) is constant. Smaller ion - size leads to stronger ionic bonds and higher lattice energy. We want to find a halide ion that, when combined with $Cs^+$, gives a similar lattice energy to lithium iodide. Since $Cs^+$ is large, we need a large halide ion.

Step2: Analyze halide ion sizes

Among fluoride ($F^-$), chloride ($Cl^-$), bromide ($Br^-$), and iodide ($I^-$), iodide ($I^-$) is the largest halide ion. Larger ions have weaker electrostatic attractions over longer distances. Since $Cs^+$ is a large cation, combining it with a large $I^-$ ion will result in a lattice energy similar to that of $Li^+$ (a small cation) combined with a large $I^-$ ion due to the relative ion - size relationships.

Answer:

Iodide ($I^-$)