In my book , the following line has been written:-

All the alkali metal hydrides are ionic solids with high melting points
The ionic character , however, decreases from Li to Cs The reason being that as the size of the cation increases , the anion hydride ion can polarise the cation more easily. As a result , the covalent character increases and hence the ionic character decreases.

Now my doubt is that , is Fajan's rule violated here? Because Fajan's rule states that for bigger sized cations, the corresponding covalent character should be less , and therefore the ionic character should be more.. but why is this not the case here as well? Why does the anion hydride ion polarise the cation more easily?

Moreover the book also says that Lithium will be the most reactive towards Hydrogen though it does mention that the reaction takes place at considerably higher temperature (1073K) compared to other alkali metals (673K).. Could anyone explain why lithium should be the most reactive metal (if it is so) despite the above given fact?


1 Answer 1


First and foremost, hydrides of alkali metals are ionic in nature. So, you cannot use Fajan's rule to determine ionic character as it is only used for covalent compounds. For ionic compounds we use something called cation-anion radius ratio RC/RA (C = cation, A=anion). For small cations like $\ce{Li+}$, small anions like $\ce{H-}$ are preferred. This is because if the cation is small and the anion is large, the electron clouds of both don't overlap enough for a strong electrostatic attraction to form. This can lead to unstable lattice energy. Hence, as we go from top to bottom in the first group of the periodic table, the size of cations increases thereby decreasing the stability(ionic character) of their respective halides

  • $\begingroup$ Welcome to ChemSE! Please find guidance for MathJaX / $\LaTeX$ as follows and reformat your post - Basics // MathJaX and mhchem formatting for equations and chemical formulae // Guidelines on which should be upright / italics $\endgroup$ Mar 27 at 14:07
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    $\begingroup$ Thanks a lot! Just one more question... For a stable lattice energy , or in other words , for more ionic character , the sizes of cation and anion must be somewhat similar? $\endgroup$
    – Adhway
    Mar 27 at 16:32
  • $\begingroup$ And... you also mentioned that halides of alkali metals are ionic in nature but isnt LiCl and lower halides have a predominant covalent character? $\endgroup$
    – Adhway
    Mar 27 at 16:42
  • $\begingroup$ Typo! I meant hydrides not halides. Really sorry $\endgroup$
    – Baksish
    Mar 28 at 6:33
  • $\begingroup$ Yes, we majorly use the relative size of anions and cations $\endgroup$
    – Baksish
    Mar 28 at 6:37

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