Arrange the following ions in ascending order of radius :

Na⁺ , F^- , O^2- , Mg²⁺

To arrange the ions Na⁺, F^-, O^2-, and Mg²⁺ in ascending order of their ionic radii, we need to consider a few key concepts about ionic size and the factors that influence it.

Understanding Ionic Radius

The ionic radius is the measure of an atom's ion in a crystal lattice. Several factors affect the size of an ion:

• Charge: Cations (positively charged ions) are generally smaller than their neutral atoms because they lose electrons, resulting in a greater effective nuclear charge per electron. Anions (negatively charged ions), on the other hand, are larger than their neutral atoms due to the addition of electrons, which increases electron-electron repulsion.
• Electron Configuration: Ions with the same electron configuration (isoelectronic species) can be compared directly. The more protons in the nucleus, the stronger the attraction on the electrons, leading to a smaller radius.

Analyzing the Given Ions

Let's look at the ions in question:

• Na⁺: Sodium loses one electron to form Na⁺, resulting in a smaller radius compared to neutral Na.
• F^-: Fluorine gains one electron to form F^-, which increases its radius compared to neutral F.
• O^2-: Oxygen gains two electrons to form O^2-, leading to a larger radius due to increased electron-electron repulsion.
• Mg²⁺: Magnesium loses two electrons to form Mg²⁺, resulting in a smaller radius than neutral Mg.

Comparing Ionic Sizes

Now, let's compare their sizes based on the factors discussed:

• Mg²⁺ has the smallest radius because it has lost two electrons and has a high nuclear charge (12 protons) attracting fewer electrons (10).
• Na⁺ is next, as it has lost one electron (11 protons attracting 10 electrons).
• F^- comes next; it has gained one electron (9 protons attracting 10 electrons), resulting in a larger radius than Na⁺.
• O^2- has the largest radius because it has gained two electrons (8 protons attracting 10 electrons), leading to significant electron-electron repulsion.

Final Arrangement

Putting it all together, the ions in ascending order of radius are:

1. Mg²⁺
2. Na⁺
3. F^-
4. O^2-

This order reflects the balance between nuclear charge and electron repulsion, which ultimately determines the size of the ions. Understanding these principles helps clarify why ionic radii vary among different ions.