Q.The ionic radii of F⁻ and Na⁺ are 133 and 98 pm, respectively.

Step 1. Check electron count. F⁻ has $9+1=10$ electrons; Na⁺ has $11-1=10$ electrons — the two are isoelectronic species (same electron count).

Step 2. Recall the isoelectronic-species rule. Among isoelectronic species, the one with the LARGER actual nuclear charge has the SMALLER radius, since a stronger nuclear pull draws in the same-sized electron cloud more tightly (section 7.5.2.2).

Step 3. Compare nuclear charges. F's nuclear charge is +9; Na's is +11. Since Na's is larger, Na⁺ should be smaller than F⁻. …

• Assuming a cation is always smaller than any anion in general, rather than checking the specific isoelectronic-species reasoning (same electron count, compare nuclear charge). …