Chemical Bonding! (3)
Chemistry
Level
3
Which Alkali Metal Iodide Has highest melting point?
Lithium
Rubidium
Caesium
Potassium
Sodium
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1 solution
Normally when the anions are polarizable the melting point order is decided by Fajan's rule ie the percentage ionic character is directly proportional to melting point.
Iodide being a big anion is easily Polarisable and lithium having very high polarising power hence Lithium iodide has highest covalent character and hence it has lowest melting point among all alkali metal halides except fluoride due to non polarisability of fluoride ion.
Next comes the interesting part I- Is so easily Polarisable that even Na+ can polarise it to some extent and hence can have some covalent character . so its melting point becomes lesser than Potassium Iodide.
The order is KI>NaI>RbI>CsI>LiI