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Na+, Mg2+, Al3+ and Si4+ are isoelectronic. The order of their ionic size is

1. Na+ > Mg2+ > Al3+ < Si4+
2. Na+ < Mg2+ > Al3+ > Si4+
3. Na+ > Mg2+ > Al3+ > Si4+
4. Na+ < Mg2+ > Al3+ < Si4+

Correct answer: Na+ > Mg2+ > Al3+ > Si4+

Solution

Isoelectronic species have the same number of electrons, but their ionic size decreases with increasing nuclear charge due to stronger attraction between the nucleus and electrons. Thus, the order is Na+ > Mg2+ > Al3+ > Si4+.

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