Lithium fluoride (LiF) and lithium iodide (LiI) each contain a single Li–X bond in the gas phase. Spectroscopic studies give the following data.
Molecule
Bond length / pm
Dipole moment / D
Melting point / °C
LiF
156
6.33
845
LiI
238
6.27
469
Constants: 1 D = 3.34×10−30C⋅m; e=1.60×10−19C.
(a) For each molecule, calculate the theoretical dipole moment for full electron transfer.
(b) Compute the percent ionic character of the Li–F and Li–I bonds.
(c) Although the percent ionic characters are very similar, the melting points of the two ionic solids differ by nearly 400 °C. Provide two distinct factors that account for this substantial difference in melting point.
(d) A student claims that the similarity of the percent ionic characters proves lattice energy is identical for LiF and LiI. Evaluate the validity of this claim, citing evidence from your calculations and from periodic trends.