As you have noted, there are two factors in play here. The electronic configuration of Na+ is the Ne core while for Ca+2 it is the Ar core. That should make Ca+2 larger. But Ca+2 has two positive charges pulling the electrons in, while Na+ only has one. Since the observed radii are nearly the same, we can conclude that the charge effect is indeed large enough to cancel the increase due to the extra shell.

Dear Roger E. Cramer Sir,

Thank you for your answer. Yes, the data shows a suitable explanation by an effective nuclear charge.

But sir, here that factor contracts two outermost shells, not the one! Another important factor, Na+ belongs to 2nd row and Ca+2 belongs to 3rd row of the periodic table - despite that Ca+2 has a smaller radius.

I am looking for an alternative explanation if any.

Sir, can you kindly cite any literature or similar trend in the periodic table- it will be of great help.

To quote Frank T Edelmann, Dear Mostofa Shamim I think that Roger E. Cramer already provided a perfect answer.

But I would like to add that this trend in the periodic table continues accross the rows while same electron shells are being added to (versus starting a new shell) while moving to the right, the radii generally decrease.

w.s.l. Boyer Frank T. Edelmann Roger E. Cramer Sir,

Thank you for your input.

But have you noticed that third-row element with a lower radius than a second-row element?

I put forward this question because of such abnormality and asking a validation (with support from literature and/or books) for the power of effective nuclear charge.

The aim of the question was not having an answer, but finding some literature mentioning such strength of effective nuclear charge.

I am sorry if I was not clear in question description section.