Periodic classification of elements


Which of the following is the smallest in size??
N, O, F or Na


The important thing to realize here is that all those ions are isoelectronic, which means that they share the same number of electrons, and thus the same electron configuration. More specifically, all those ions have the electron configuration of neon, Ne , which is a noble gas.

Had we have had any charges specified to each of these elements, we would have been able to order then and/or select the smallest element based on the size. But as none of them have any effective nuclear charge specified here, it’s a bit difficult (or not possible to in other term) to say anything on this end.


que error​:sweat_smile::sweat_smile:


In which case it’ll be as follows:

N3- > O2- > I- > Na+

So, to answer your question, the smallest one among the mentioned element is ‘Na+’.

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, they’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 couldn’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+