Discussion on: Periodic classification of elements

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In which case itโ€™ll be as follows:

N3- > O2- > I- > Na+

So, to answer your question, the smallest one among the mentioned element is โ€˜Na+โ€™.

As explained a bit above, the atomic and ionic size is determined by the distance from the nucleus to the outermost electrons. But for all those elements, theyโ€™ve the outermost electrons on the same energy level which is n = 2. This forces us to look into effective nuclear charge. That is also why I couldnโ€™t give specific answer with no nuclear charge being mentioned earlier.

The attraction between the outermost electrons and the nucleus is what determines the size of the ion here. This implies that the more positive the nucleus is, the more attracted the outermost electrons will be. In this case, the nucleus with the most protons will compress the energy levels of the electrons the most, which will result in the ion having the smallest size.

This means that ionic size will increase in the order Na+, I-, O2-, N3-. Since, Sodium (Na) has the biggest atomic number and Nitrogen (N) the smallest. Therefore the order is as highlighted above, but placed here again for easier reference:

N3- > O2- > I- > Na+

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