What are the factors that affect the lattice energy, the amount of energy needed to break an ionic solid into its constituent ions, both in the gas phase? The absolute first thing that you need to consider is ion charge. Here in the table, we have a list of the lattice energies when you pair up each of the cations with each of the anions. You'll notice that the highest value in each row corresponds to O minus 2. That has a charge of 2 minus, and so those ions are doubly as attracted to positive ions as minus 1 ions, simply because they have double the charge. And you'll notice the lattice energy for that higher charged ion is the highest in its entire row.
You can also see that's the case for cations. We've got some plus one ions, some plus two ions, and a plus three ion. The plus three ion always has the highest lattice energy of its column.
So bigger charges mean bigger lattice energies always. If there's a tie, as in if you're intercomparing two ions that happen to have the same charge, then it comes down to ion size. Smaller ions. Smaller ions have a higher lattice energy. The trend here is that fluoride is way smaller than iodide.
And I mean, that continues through the halogens, right? Smallest, next smallest, kind of big, biggest. And you'll notice that the highest value in that row happens to correspond to the smallest ion with that charge, of course. True here, true here, true here, true here, true here, and true here. Almost not true here.
This one actually ends up being very close to a tie for all of them, presumably because it's the plus three ion that they have in common that really makes the main difference. In any case, compare the ion charges. Bigger charges lead you to a higher lattice energy.
Then, if there's a tie, you can compare ion sizes. Smaller ions will generally give higher lattice energies. Capisce?
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