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My textbook mentions that across a period from left to right, the ionic radius of atoms decreases. I'm a bit puzzled because negative ions in the same period have an extra shell. I get the part about atomic radii decreasing from left to right due to the effective nuclear charge, but how is it that negative ions with an additional shell end up having a lower ionic radius than positive ions with a whole shell less?

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TL;DR

Basically there are two possibilites:

  1. Either the book is incorrect, or
  2. The book is talking about isoelectronic species in a confusing manner.

The concept: The approach to studying atomic and ionic radii differs, as we focus on isoelectronic species rather than group or period trends. Ionic radii decreases as the charge decreases, while in a group, they increase. For example, the 2nd period of elements Li, Be, B, C, N, O, F, and Ne shows trends in ionic radii. The first three elements lose electrons and gain He configuration, while $\ce{C}$ gains electrons and becomes a negative ion, larger than the positive ions. The last few elements show a decrease in radii.


Glossary

Isoelectronic species: Species having the same number of electrons. (e.g.) $\ce{N^3-, O^2-, F-, Ne, Na+, Mg^2+, Al^3+}$


Background note:

There is a difference in the approach when we generally talk about atomic and ionic radii.

We don't discuss group or period trends for ionic radii - rather, we talk about isoelectronic species and remember the fact that for isoelectronic species - as the charge decreases, the ionic radius decreases.

Obviously in a group the ionic radii will increase (as a shell is added.)


Now coming to your question:

What are the trends in ionic radii across a period?

Let us take the 2nd period.

Its elements are $\ce{Li, Be, B, C, N, O, F}$ and $\ce{Ne}$.

Now the first 3 would want to lose their electrons and gain He configuration. Compared to $\ce{H-}$ they are going to be smaller.

As we go across the period: $\ce{C}$ is like the cat on the wall. It could lose or gain 4 electrons, but both of them are quite unstable and thus it would share its electrons.

But here's the kicker: when it gains electrons and becomes a negative ion, it's larger than the positive ions before it. Why? The added electrons introduce some repulsion that slightly counteracts the nuclear pull. And if $\ce{C}$ loses its electrons - it will become way smaller than $\ce{Li+}$, $\ce{Be^2+}$ and $\ce{B^3+}$.

As we go further, the last few elements of 2nd period - we can see a decrease in the radii. So the graph of ionic radii will look like: enter image description here


Now THIS is the reason - this nitty-gritty trend - that we usually restrict ourselves (atleast at my level) to studying isoelectronic species.

The graph will look different if $\ce{C^4−}$ is taken. The peak will occur at $\ce{C}$ itself.


Sources for image: https://chart-studio.plotly.com/~hannahghutson/14.embed

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