2 questions about MgO n NaF?

Question

the question is:
MgO and NaF is iso-electronic.However,the lattice energy of MgO is almost four times that of NaF.This is because:
1.The charges on the ions in MgO are higher than the charges on the ions in NaF
2.The inter-ionic distance in MgO is shorter as compared with that in NaF
3.The hydration energy of the Mg2+ and O2- ion is more exothermic.

My teacher told me that the (1) ,(2) are true.but (3) is false.
I really dont get it why (2) is true,because at that time ,it is still an exam,so we only got a sheet of table periodic with us.And from it,i can just see that Na 's radius is larger than Mg,but O's radius is larger than F.
I dont know which one between MgO n NaF has shorter distance of interionic.
Can somebody tell me how to see that by just using table periodic or maybe some theory?
and also,I can't think why (3) is wrong..

Answer 1

Firstly Hydration energy and lattice energies are indirectly related
SOLUBILTY is the relating factor betweeen the latter and the former.
For a substance to dissolve the hydration energy must exeed the lattice energy.
So option 3 is ruled out.

Since Mg cation has more charge double of Na cation, lattice energy being directly proportional is more too ( from Borne-Landes equation ).
So option 1 is correct.

Donot compare the atomic radii, see their ionic radii.
but since charge density is more in MgO than NaF the interionic packing is more compact and thus the inter-ionic distance is shorter too.
Thus option 2 is also correct.

Your teacher was right afterall.