What could we say about the structure of an ionic salt composed of magnesium, and nitrogen ions?
Not much at all.
We would assume that the given elements form an ionic salt, i.e.
The nitrogens are not jointed together by bonds between the nitrogens. The shape is a complex of ions.
One magnesium will be joined to the first nitrogen with a double bond formed by the Magnesium becoming +2 and the Nitrogen becoming -3. The second Magnesium will be joined to both Nitrogens sharing one electron with each of the two Nitrogens.
The third Magnesium like the first with be joined to the second Nitrogen with a double bond formed by the Magnesium becoming +2 and the Nitrogen -3
Most ionic solids like this compound form crystal ionic structures with multiple ions combining to form a three dimension complex of positive and negative ions.