The statement(s) that are correct about the species O²⁻, F⁻, Na⁺ and Mg²⁺.
(A) All are isoelectronic
(B) All have the same nuclear charge
(C) O²⁻ has the largest ionic radii
(D) Mg²⁺ has the smallest ionic radii
Choose the most appropriate answer from the options given below :

Solution

Chemistry ma’am · 4 steps

Same 10 electrons in every ion, different protons pulling on them: more protons means a smaller ion.

1. Count electrons. O has 8 electrons and gains 2, F has 9 and gains 1, Na has 11 and loses 1, Mg has 12 and loses 2. Each ion ends up with 10 electrons, so (A) is correct.
2. Count protons. The nuclear charge is the atomic number, and gaining or losing electrons does not change it. These are four different elements with Z from 8 to 12, so (B) is wrong.

   Ion	Protons (Z)	Electrons	Ionic radius
   O²⁻	8	10	largest, about 140 pm
   F⁻	9	10	about 133 pm
   Na⁺	11	10	about 102 pm
   Mg²⁺	12	10	smallest, about 72 pm

3. Compare sizes. All four have the same 10 electrons, so the nucleus with more protons pulls them in harder. O²⁻ has the fewest protons and is the largest. Mg²⁺ has the most protons and is the smallest. So (C) and (D) are correct.
4. So. (A), (C) and (D) are correct and (B) is wrong, which is option (b).

The idea to keep. In an isoelectronic series the electron count is fixed, so size depends only on nuclear charge. More protons give a smaller ion, and more negative charge gives a bigger one.

Answer from the NTA final answer key. Solution by PrepWiser.