Among the compounds with greatest and least ionic character respectively are

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Accepted Answer

The ionic character of a compound is inversely proportional to its covalent character. According to Fajan's rules, covalent character increases with:

Small size of the cation.
Large size of the anion.
Large charge on either ion.
Pseudo noble gas configuration of the cation (less relevant here).

We are comparing $L i C l$ , $R b C l$ , $B e C l_{2}$ , and $M g C l_{2}$ . All have the same anion, $C l^{-}$ . Therefore, we need to compare the cations: $L i^{+}$ , $R b^{+}$ , $B e^{2 +}$ , and $M g^{2 +}$ .

Step 1: Compare ionic character based on cation size for monovalent ions.

For $L i C l$ and $R b C l$ , both cations are monovalent. $R b^{+}$ is much larger than $L i^{+}$ (down the group in alkali metals). A larger cation leads to less polarizing power, hence more ionic character. Therefore, $R b C l$ is more ionic than $L i C l$ .

Step 2: Compare ionic character based on cation size and charge for divalent ions.

For $B e C l_{2}$ and $M g C l_{2}$ , both cations are divalent. $M g^{2 +}$ is larger than $B e^{2 +}$ (down the group in alkaline earth metals). A larger cation leads to less polarizing power, hence more ionic character. Therefore, $M g C l_{2}$ is more ionic than $B e C l_{2}$ .

Step 3: Compare overall ionic character.

Now, let's compare all four. The charge on the cation plays a significant role. A higher charge on the cation increases its polarizing power, leading to more covalent character. Thus, compounds with $+ 2$ cations ( $B e C l_{2}$ , $M g C l_{2}$ ) will generally be less ionic (more covalent) than compounds with $+ 1$ cations ( $L i C l$ , $R b C l$ ).

Among $+ 1$ cations, $R b^{+}$ is the largest, so $R b C l$ has the greatest ionic character.

Among $+ 2$ cations, $B e^{2 +}$ is the smallest and has a high charge density, making it highly polarizing. Therefore, $B e C l_{2}$ has the greatest covalent character, and thus the least ionic character.

Arranging in decreasing order of ionic character:

$R b C l > L i C l > M g C l_{2} > B e C l_{2}$

Therefore, the compound with the greatest ionic character is $R b C l$ , and the compound with the least ionic character is $B e C l_{2}$ .

Option Analysis:

A) $L i C l, R b C l$ : Incorrect. $L i C l$ does not have the greatest ionic character, nor does $R b C l$ have the least.
B) $R b C l, B e C l_{2}$ : Correct. $R b C l$ has the greatest ionic character and $B e C l_{2}$ has the least ionic character.
C) $R b C l, M g C l_{2}$ : Incorrect. $M g C l_{2}$ does not have the least ionic character.
D) $M g C l_{2}, B e C l_{2}$ : Incorrect. $M g C l_{2}$ does not have the greatest ionic character.

Correct Answer: (B)

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