Q.
Al3+ has a lower ionic radius than Mg2+ ion because
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a
Mg atom has less number of neutrons than Al
b
Al3+ has a higher nuclear charge than Mg2+
c
Their electronegativities are different
d
Al has a lower ionisation potential than Mg atom.
answer is B.
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Detailed Solution
As we can see here that both the species are isoelectronic species,as they all contain the same number of electrons i.e,10.
And we know that for isoelectronic species size depends upon the effective nuclear charge, more is the positive charge,more is the effective nuclear charge and thus ,lesser is the size.
Hence, has a lower ionic radius than ion because it has a higher nuclear charge than .
Hence, option(b) is correct.

