Q.21
The ions with lowest and highest radii among
O2−, F−, Na+ and Mg2+
are respectively,
Options:
(A) Mg2+ and O2−
(B) O2− and F−
(C) O2− and Mg2+
(D) Na+ and Mg2+
Lowest and Highest Ionic Radii Among O2−, F−, Na+ and Mg2+
Ionic radius questions are based on periodic trends
and the concept of isoelectronic species.
In this solution, we identify the ions with the
lowest and highest radii.
Key Concept: Isoelectronic Species
All the given ions have the same number of electrons (10),
hence they are isoelectronic.
| Ion | Atomic Number | Charge | Electrons |
|---|---|---|---|
| O2− | 8 | −2 | 10 |
| F− | 9 | −1 | 10 |
| Na+ | 11 | +1 | 10 |
| Mg2+ | 12 | +2 | 10 |
Order of Ionic Radii
In an isoelectronic series, ionic radius decreases with
increasing nuclear charge.
O2− >F− >Na+ >Mg2+
Largest radius: O2−
Smallest radius: Mg2+
Correct Answer
Option (A): Mg2+ and O2−
Conclusion
In an isoelectronic series, ionic size decreases as nuclear charge increases.
Therefore, among the given ions:
Smallest ion = Mg2+
Largest ion = O2−
Hence, the correct answer is Option (A).
