A-Level Bonding Question Help!

May someone please tell me the answers and the reasoning behind the the answers? Thankyou!

13 The ionic radii in nm of some ions are given below.
Li+ 0.074 F– 0.133
Ca2+ 0.100 Cl– 0.180
O2– 0.140
S2– 0.185
(a) Which of the following compounds has the most exothermic lattice energy? They all have the same crystal structure.


A LiF
B LiCl
C CaO
D CaS
(b) Which of the following compounds will show the greatest difference between the
experimental (Born-Haber) lattice energy and that calculated from a purely ionic
model?
A LiF
B Li2O
C CaO
D CaS

Hey. Sneaky question I must say, it does require a bit of thinking.

Ok so for part A : The answer is c. The question asks us which of the compounds has the most exothermic lattice energy. Now, consider this. 'When bonds are made, energy is released due to the attraction between the opposite charges ( be it of electrons and nuclei or anions and cations ). The stronger the bond, the more energy is released. ' So we just have to look at which of the compounds have the strongest electrostatic attractions between them. As Ca2+ has a higher charge density than Li+ it has to be a compound with Ca2+. Now as O2- has a smaller ionic radius than S2-, the Ca2+ amd the O2- can get closer together in the crystal structure, forming a stronger bond, hence releasing more energy. So the answer is C .

Ok so for Part B : Now we are asked which of the compounds will will show the greatest difference between the experimental (Born-Haber) lattice energy and that calculated from a purely ionic model? The question is basically asking us, which of the compounds shows the most covalent character. It will have to be the compound with the highest charge density in the cation ( so Ca2+ ) and the one with the largest ionic radius in the anion. As S2- has a larger ionic radius than O2- ( as we are told in the question ) the electrons of the S2- will be more polarized by the Ca2+, which will result in a larger difference between the two values. SO the answer is D.

Hope this helps, and sorry about the long answers, I just wanted to make sure you understood why the answers were C and D.

Good luck

Thankyou so much! Really appreciate it So although a compound may show more covalent character than its pure theoretical ionic model, it doesn't have to mean it will have a more exothermic lattice energy?

If a a compound shows more covalent character it's lattice enthalpy by born harber cycle will be more exothermic than lattice enthaply by pure ionic model. This however doesn't mean that is will have a more exothermic lattice. Like for example, take beryillium iodide. It has some covalent character, but that doesn't mean that is has a more exothermic lattice energy than another compound like for example beryillium flouride. How exothermic lattice energy of a compound is comes down to charge density of the ions and sizes, not whether or not the compound has covalent character.

Thankyou for clearing that up! you clever guy