lattice energy & atomic radius

[sacha]

in one of the TBR passages (pg170), the formula for lattice energy is given as
E = (Q of cation x Q of anion)/r
r is defined as distance between ions & Q is charge

For those who have the book, i feel like qu 15 & 21 given conflicting answer explanations for this passage

qu15. if sodium is used instead of lithium, lattice energy would decrease.
explanation: since sodium cation has a larger atomic radius than lithium cation, lattice energy of sodium would be less due to a larger r value in the denominator

q21.what changes would reduce the lattice energy? substituting a larger cation for a smaller cation
explanation: substituting a larger ion for a smaller ion increases the distance between ions (r) & decreasing lattice energy

the first question substitutes a smaller cation with a bigger one while the other substitutes a larger cation for a smaller one but both result in a decrease in lattice energy? which is right? the second explanation makes more sense to me..

is this a mistake in the book? or am I making an error somewhere?

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[Rabolisk]

You are misreading the second sentence. Substituting a larger cation for a smaller cation means that you are taking out a smaller cation and replacing it with a larger cation. English not your strong suit? Though, I admit, it happens to all of us, especially when we are trying to comprehend new information.

 

[sacha]

You are misreading the second sentence. Substituting a larger cation for a smaller cation means that you are taking out a smaller cation and replacing it with a larger cation. English not your strong suit? Though, I admit, it happens to all of us, especially when we are trying to comprehend new information.

lol. i just realised I misread that. brain-dead moment. thanks!