(a) The species N3−, O2−, F−,Na+ in their increasing order of ionic radii:
Na+ < F− < O2− < N3−
(b) Across the period-Atomic radii decrease on account of the following facts: Nuclear charge increases progressively by one unit but the additional electron goes to the same principal shell. As a result, the electron cloud is pulled closer to the nucleus by the increased effective nuclear charge.
Across the group: Atomic radii increase
Reason: Nuclear charge increases with increase in atomic numbers but at the same time, there is progressive increase in principal energy levels.