Why are hydrated lithium ions' radii larger than hydrated sodium ions' radii i.e. $r_\ce{Li+(aq)}>r_\ce{Na+(aq)}$?

If ionic radii increase down the group i.e. $r_\ce{Li+}<r_\ce{Na+}<r_\ce{K+}$ then the radii of hydrated ions also increase down the group i.e. $r_\ce{Li+(aq)}<r_\ce{Na+(aq)}<r_\ce{K+(aq)}$. But according to my class notes, The radii of hydrated ions are larger than those of gaseous ions. The radii of hydrated ions decrease as the group. I'm not sure I understand. But, according to the class notes and Quora answers, how can the size of an ion alter the hydration radii?

According to a Quora answers,

Hydration energy is the amount of energy liberated when an ion gets hydrated that is , it accepts water molecule. e.g. lithium ion is small in size so it accepts large number of water molecule so it's hydration energy is more whereas caesium ion is bigger in size so it accepts less number of water molecule and has less hydration energy.

Another answer says,

As the size of ion increases its polasiation tendency decreases therefore the tendency to polarise nearby water molecules decreases hence decreasing its hydration radii. This effect can be seen by observing the trend of hydartion radii in alkali metal ions.

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    $\begingroup$ Look around the site, it was discussed recently. $\endgroup$
    – Poutnik
    Commented Dec 19, 2021 at 15:37
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    $\begingroup$ Please check you greater/smaller than signs. Are they like you intended? $\endgroup$ Commented Dec 19, 2021 at 18:55
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1 Answer 1


Down a group, ionic radii increases. But at the same time, due to decrease in charge density, the ions attract less number of water molecules. The smallest cation (with the same charges) having highest charge/size ratio attracts maximum number of water molecules.

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Image credit: https://rwu.pressbooks.pub/bio103/chapter/the-chemical-context-of-life/

Compare the relative increase in ionic radii down a group to effective radii of hydrated ion, i.e., with attached whole water molecules. Clearly, there will be a decrease in radii of hydrated ions.


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