Arrange these ions according to ionic radius.
Largest radius
Smallest radius
N³-
Mg²+
F-
Na+
O²-
The Correct Answer and Explanation is:
Correct Order of Ions by Ionic Radius (Largest to Smallest):
Largest Radius → Smallest Radius:
N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺
Explanation
Ionic radius refers to the size of an ion. The size of an ion depends on two main factors:
- The number of electrons relative to protons, and
- The effective nuclear charge (Z_eff)—the net positive charge experienced by an electron in a multi-electron atom.
Let’s examine each ion:
- N³⁻, O²⁻, and F⁻ are all anions (negatively charged). They gain electrons, increasing electron-electron repulsion and expanding the electron cloud, making them larger than their neutral atoms.
- Na⁺ and Mg²⁺ are cations (positively charged). They lose electrons, reducing electron repulsion and allowing the nucleus to pull remaining electrons closer, thus decreasing the ionic radius.
All five ions are isoelectronic—they all have the same number of electrons (10), but different nuclear charges (number of protons):
| Ion | Nuclear Charge (Protons) | Ionic Charge | Relative Size |
|---|---|---|---|
| N³⁻ | 7 | -3 | Largest |
| O²⁻ | 8 | -2 | Larger |
| F⁻ | 9 | -1 | Large |
| Na⁺ | 11 | +1 | Small |
| Mg²⁺ | 12 | +2 | Smallest |
Despite having the same number of electrons, N³⁻ has the fewest protons, so its nucleus exerts the weakest pull on its electrons, making its radius the largest. In contrast, Mg²⁺ has the most protons, pulling the same number of electrons in more tightly, resulting in the smallest radius.
Thus, as nuclear charge increases among isoelectronic species, ionic radius decreases.
This is why the order is:
N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺.
