Q14.30 P - Chemistry: Molecular Nature Of Matter And Change

Question

Which member of each pair has more ionic character in its bonds: 

(a) BeF₂ or CaF₂; (b) PbF₂ or PbF₄; (c) GeF₄ or PF₃?

Step-by-Step Solution

Answer

The compound with more ionic character in its bonds is shown below:

(a) CaF₂

(b) PbF₂

(c) GeF₄

Step 1: Electronegativity difference

Electronegativity refers to the chemical process that determines the tendency of an atom involved in an ionic and covalent bond to attract the bonding electron towards itself. 

If the electronegativity difference between the two bonded atoms is smaller, it results in a covalent bond. In contrast, if the electronegativity difference between the two bonded atoms is larger, it results in an ionic bond with a more ionic character.

The symbol denotes the electronegativity difference, $\mathbf{\Delta }\mathit{\chi }$and it is given by:

$\mathbf{\Delta }\mathit{\chi }\mathbf{ }\mathbf{=}\mathbf{ }{\mathit{\chi }}_{\mathbf{2}}\mathbf{-}{\mathit{\chi }}_{\mathbf{1}}$

where ${\mathit{\chi }}_{\mathbf{1}}$ and ${\mathit{\chi }}_{\mathbf{2}}$ represents electronegativity of the first and second compound respectively.

Step 2: Fajan’s rule

Fajan’s rule is used in inorganic chemistry to predict the bond character, covalent or ionic in the chemical bond. The conditions that favour the ionic character in a compound depend upon the size of the cation and the anion.

• Lower polarising power of the cation results in more ionic character in a chemical bond.
• Larger size of cation and smaller size of anion contributes to more ionic character in the chemical bond.

Step 3: Identify the compound with more ionic character in each of the following given pairs:

(a) CaF₂ will show more _­ionic character than BeF₂ because of the smaller size of Be⁺ as compared to the Ca⁺ ion. Therefore, polarisation occurs more in BeF₂ as compared to CaF_2. Thus, it becomes less ionic than CaF_2; as we know, when polarisation increases, ionic character decreases.

 

(b) PbF₂ has more ionic character than PbF_4, because increasing the oxidation number on the central atom decreases the ionic character. PbF₄ has a +4 oxidation state, whereas PbF₂ has a +2 oxidation state on its central atom.

 

(c) GeF₄ will show more ionic character than PF_3, because the electronegativity difference of GeF₄ is more as compared to PF_3. Thus, it has more ionic character in its chemical bonds.

 $$\begin{gathered} \Delta \chi Ge{F}_4 = (4\times 3.98)-2.01 \\ = 15.98-2.01 \\ = 13.91 \end{gathered}$$> $$\begin{gathered} \Delta \chi P{F}_3 = (3\times 3.98)-2.10 \\ = 11.94-2.01 \\ = 9.84 \end{gathered}$$