JEE MAIN - Chemistry (2010 - No. 24)
The correct sequence which shows decreasing order of the ionic radii of the elements is
Al3+ > Mg2+ > Na+ > F- > O2-
Na+ > Mg2+ > Al3+ > O2– > F–
Na+ > F– > Mg2+ > O2– > Al3+
O2– > F– > Na+ > Mg2+ > Al3+
Explication
As you can see all of them are isoelectronic and for isoelectric elements which ion has more positive charge will have lesser size. And which ion has more negative charge that ion's size will be more.
So, the correct order is
$${O^{2 - }} > {F^ - } > N{a^ + } > m{g^{2 + }} > A{l^{3 + }}$$
So, the correct order is
$${O^{2 - }} > {F^ - } > N{a^ + } > m{g^{2 + }} > A{l^{3 + }}$$