Question

Why is the distance between the ions smaller in MgO than in NaF?

Answer 1

Surely you have got a text, or lecture notes?

Explanation:

Atomic size decreases across the Period, from left to right as we face the Table....and this is the effect of increased nuclear charge and ineffective electronic shielding. And thus `Mg^(2+)`, a dication, should be smaller than `Na^+`, a singly charge cation. And by the same reasoning, `O^(2-)`, a dianion, should be LARGER than `F^-`, a monoanion.

The electrostatic interaction should be greater in `MgO`, and this should be reflected in melting points, and solubility data....and this should also be reflected in reduced interatomic distances, the which, as a chemist, you should look up and quote.