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Question:

The correct sequence which shows the decreasing order of the ionic radii of the elements is:

Al3+>Mg2+>Na+>F>O2−

Na+>F−>Mg2+>O2−>Al3+

Na+>Mg2+>Al3+>O2−>F−

O2−>F−>Na+>Mg2+>Al3+

Solution:

For isoelectronic species, higher the Z/e ratio, smaller is the ionic radius.
O2−=8/10=0.8
F−=9/10=0.9
Na+=11/10=1.1
Mg2+=12/10=1.2
Al3+=13/10=1.3
Therefore, the decreasing order of ionic radii is O2−>F−>Na+>Mg2+>Al3+