(D) Na⁺ > F⁻ > Mg²⁺ > Al³⁺

Introduction

Understanding ionic size trends is crucial in inorganic chemistry and is frequently tested in JEE, NEET, and other competitive exams.
When different ions have the same number of electrons, their sizes depend mainly on nuclear charge.

In this article, we determine the correct ionic size order of Na⁺, F⁻, Mg²⁺, and Al³⁺ and explain why only one option is correct.

Correct Answer

Option (B): F⁻ > Na⁺ > Mg²⁺ > Al³⁺

Key Concept: Isoelectronic Species

All the given ions have 10 electrons, same as Neon (Ne).

👉 Such species are called isoelectronic ions.

Rule for Isoelectronic Series

✔ More nuclear charge → stronger attraction → smaller ionic size
✔ Less nuclear charge → weaker attraction → larger ionic size

Correct Ionic Size Order

Nuclear charge increases as:
F⁻ < Na⁺ < Mg²⁺ < Al³⁺

So, ionic size decreases in the same order:

F⁻ > Na⁺ > Mg²⁺ > Al³⁺

Option-wise Explanation

Option (A): Al³⁺ > Mg²⁺ > Na⁺ > F⁻

 Completely incorrect

• Al³⁺ has the highest nuclear charge

• Should be the smallest ion, not the largest

Option (B): F⁻ > Na⁺ > Mg²⁺ > Al³⁺

Correct Answer

• Lowest nuclear charge → largest size (F⁻)

• Highest nuclear charge → smallest size (Al³⁺)

✔ Follows isoelectronic rule perfectly

Option (C): Al³⁺ > Mg²⁺ > F⁻ > Na⁺

 Incorrect

• Reverses the correct nuclear charge trend

• Al³⁺ cannot be larger than F⁻

Option (D): Na⁺ > F⁻ > Mg²⁺ > Al³⁺

 Incorrect

• F⁻ is larger than Na⁺ due to lower nuclear charge

Why F⁻ Is the Largest Ion

✔ Lowest nuclear charge
✔ Weaker attraction on electrons
✔ Larger electron cloud

Why Al³⁺ Is the Smallest Ion

✔ Highest nuclear charge
✔ Strongest attraction on electrons
✔ Highly compact ion

Final Conclusion

Since Na⁺, F⁻, Mg²⁺ and Al³⁺ are isoelectronic, their ionic sizes depend only on nuclear charge.
The ion with least nuclear charge is largest, and the one with highest nuclear charge is smallest.

Final Answer

Correct Option: (B) F⁻ > Na⁺ > Mg²⁺ > Al³⁺