The ionic radius of Na ions is 1.02 Å. The ionic radii (in Å) of Mg and Al, respectively, are:

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Strategy:
 Isoelectronic ions () decrease in size as nuclear charge () increases. Use the relationship radius  for estimation.

Step 1: Compare nuclear charges
- : , radius .
- : .
- : .

Step 2: Rank values
A higher  pulls the 10 electrons tighter. Therefore, the radius of  must be less than , and  must be the smallest.
Standard values:  and .

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