Concept:For purely isoelectronic species (ions with the same number of electrons), the ionic radius is inversely proportional to the magnitude of the positive charge (or nuclear charge).
Formula:$$ \text{Ionic Radius} \propto \frac{1}{\text{Nuclear Charge (Protons)}} $$
Solution:- All given ions are isoelectronic, containing exactly 10 electrons (configuration of Neon: \( 1s^2 2s^2 2p^6 \)).
- The number of protons pulling on these 10 electrons are: Na (11), Mg (12), Al (13), Si (14).
- \( \text{Si}^{+4} \) has the highest nuclear charge (+14).
- The stronger nuclear pull draws the remaining electrons much closer, resulting in the hypothetically smallest cationic radius.
Why other options are incorrect:- The other ions have fewer protons, meaning their nucleus exerts a weaker pull on the 10 electrons, leaving their ionic radii comparatively larger than \( \text{Si}^{+4} \).
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