Na⁺, Mg²⁺, Al³⁺ and Si⁴⁺ ions are isoelectronic. The value of ionic radii of these ions would be in the order :

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${Na}^{+} > {Mg}^{2 +} > {Al}^{3 +} > {Si}^{4 +}$

${Na}^{+} < {Mg}^{2 +} < {Al}^{3 +} < {Si}^{4 +}$

${Na}^{+} > {Mg}^{2 +} > {Al}^{3 +} < {Si}^{4 +}$

${Na}^{+} < {Mg}^{2 +} > {Al}^{3 +} > {Si}^{4 +}$

answer is A.

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Detailed Solution

Species Na⁺ Mg²⁺ Al³⁺ Si⁴⁺
Protons I I 12 13 14
Electrons l0 10 10 l0
Size of isoelectronic cations decreases with increase in magnitude of nuclear charge
$∴$ Order of decreasing size is
Na⁺> Mg²⁺ > Al³⁺ > Si⁴⁺

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