Question

The statement(s) that are correct about the species O 2^– , 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 :

• A. (B), (C) and (D) only
• B. (A), (B), (C) and (D)
• C. (C) and (D) only
• D. (A), (C) and (D) only

Answer 1

The Correct Option is D

- \( O^{2-}, F^-, Na^+, \text{ and } Mg^{2+} \) are isoelectronic (same number of electrons = 10).

- \( O^{2-} \) has the largest ionic radius due to its lowest effective nuclear charge (\( Z_{\text{eff}} \)).

- \( Mg^{2+} \) has the smallest ionic radius due to its highest effective nuclear charge.
Hence, statements (A), (C), and (D) are correct.

Answer 2

Step 1: Determine electronic configurations
We have the following species:
1. \( O^{2-} : 8 + 2 = 10 \text{ electrons} \)
2. \( F^{-} : 9 + 1 = 10 \text{ electrons} \)
3. \( Na^{+} : 11 - 1 = 10 \text{ electrons} \)
4. \( Mg^{2+} : 12 - 2 = 10 \text{ electrons} \)

All of them have 10 electrons each.

Step 2: Check statement (A)
(A) All are isoelectronic. ✔ True — each species has 10 electrons and thus is isoelectronic with neon.

Step 3: Check statement (B)
(B) All have the same nuclear charge. ✖ False — their nuclear charges differ:
O (Z = 8), F (Z = 9), Na (Z = 11), Mg (Z = 12).

Step 4: Check statement (C)
(C) \( O^{2-} \) has the largest ionic radius. ✔ True — among isoelectronic species, the ionic radius decreases as the nuclear charge increases. Since \( O^{2-} \) has the smallest nuclear charge (Z = 8), it experiences the weakest attraction on its 10 electrons, giving it the largest radius.

Step 5: Check statement (D)
(D) \( Mg^{2+} \) has the smallest ionic radius. ✔ True — \( Mg^{2+} \) has the largest nuclear charge (Z = 12) among these ions, hence the strongest attraction on electrons, making it the smallest in size.

Step 6: Conclusion
The correct statements are:

(A), (C) and (D) only