Which is more covalent, AgCl, NaCl, LiCl or KCl?

 

In the above compounds, Cl^– anion is common for all, but cations are different. According to Fajan’s rule, the covalent character in ionic compounds depends on the size of cation, anion and charge of both the cation and anion.

Now, the cation with smaller size and high charge has higher polarizing power. That is, they cause polarization of electron charge cloud of an anion to a greater extent.

 

Again, the extent of polarization of anion by cation is directly proportional to the covalent character in ionic compounds.

Read More: Why is Cu²⁺ ion more stable than Cu⁺ ion in aqueous solution.

Now, Li, Na and K are group-I elements. The cationic size of them increases from Li to K along the group. That is, the size of Li⁺ is small and that of K⁺ is large. So, Li⁺ ion have high polarizing power.

But, it has been experimentally found that, cations with pseudo inert gas configuration, such as, ns² p⁶ d ¹⁰, or with inert pair configuration, such as, (n-1)d ¹⁰ ns ², have high polarizing power while cations with noble gas configuration, such as, ns² np⁶, have low polarizing power.

That is, due to greater effective nuclear charge [low shielding effect of f-orbital] in the former cases and smaller effective nuclear charge in the later cases. 

For the above reason, Ag⁺ with pseudo inert gas configuration, such as, ns² p⁶ d ¹⁰, or with inert pair configuration, such as, (n-1)d ¹⁰ ns ², have high polarizing power.

So, the order of polarizing power of the above four cations is, Ag⁺> Li⁺>Na⁺ >K⁺. Hence the descending order of covalent character of the above compounds, AgCl > LiCl > NaCl > KCl . That is, AgCl is most covalent compound.

Why is the melting point of KCl less than NaCl?

 

In both the compounds, anion is common but cation [Na⁺, K⁺] is different. Now, according to Fajan’s rule, the cation with smaller size has higher polarizing power, that is, they cause polarization of electron charge cloud of an anion to a greater extent.

Hence, with decrease in cationic size, the covalent character increases. Now, the size of Na⁺ cation is smaller than K⁺ ion. So Na⁺ cation have higher polarizing power than K⁺ cation. So, NaCl is more covalent than KCl .

Therefore, the melting point of KCl should be higher than NaCl . But, actually the order is reversed. That is, the melting point of KCl less than NaCl.

Because, lattice energy of NaCl is higher than KCl due to large size of K⁺ ion than Na⁺ ion [lattice energy is inversely proportional to the radius of cation and anion].

Hence, the amount of energy is required to break the KCl ionic bond is less than that of NaCl ionic bond. Consequently, the melting point of KCl is less than that of NaCl.

 

 covalent character AgCl, NaCl, LiCl or KCl, melting point of KCl less than NaCl

 

• Which is more covalent, AgCl, NaCl, LiCl or KCl?
• Why is the melting point of KCl less than NaCl?