Problem 29 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Explain the difference in the nature of bonding in \(\mathrm{LiF}\) and LiI.

Step-by-Step Solution

Answer

LiF is more ionic due to higher electronegativity difference, while LiI has some covalent character due to the polarizability of iodine.

Step 1: Understanding Ionic Bonds

Both \(\mathrm{LiF}\) and LiI involve ionic bonds because lithium (\(\mathrm{Li^+}\)) combines with a halide ion (\(\mathrm{F^-}\) or \(\mathrm{I^-}\)), transferring electrons which results in oppositely charged ions. These ions are held together by electrostatic attraction.

Step 2: Difference in Electronegativity

Fluorine is the most electronegative element, while iodine is far less electronegative. This means that the electronegativity difference between \(\mathrm{Li}\) and \(\mathrm{F}\) is greater than between \(\mathrm{Li}\) and \(\mathrm{I}\). As a result, \(\mathrm{LiF}\) has a stronger ionic character due to the higher electronegativity difference.

Step 3: Polarizability and Covalent Character

Iodine is larger in size compared to fluorine and is more easily polarizable. This means that the electron cloud of \(\mathrm{I^-}\) can be distorted by \(\mathrm{Li^+}\), introducing some covalent character to LiI's bond, whereas \(\mathrm{LiF}\) retains mostly ionic character because \(\mathrm{F^-}\) is less polarizable.

Key Concepts

Electronegativity Difference

Electronegativity refers to an atom's ability to attract and hold onto electrons when it is part of a compound. The greater the electronegativity, the more strongly an atom can pull electrons toward itself. When comparing compounds like \( \mathrm{LiF} \) and \( \mathrm{LiI} \), the concept of electronegativity difference becomes crucial in understanding the nature of the bond.
Elements like fluorine and iodine have differing electronegativities. Fluorine is the most electronegative element, meaning it attracts electrons very strongly. Iodine, on the other hand, is much less electronegative.
- In \( \mathrm{LiF} \), the electronegativity difference between lithium and fluorine is large. - In \( \mathrm{LiI} \), this difference is smaller because iodine's electronegativity is lower.
A larger electronegativity difference often indicates a more ionic bond, where one atom essentially "takes" electrons from another. Therefore, \( \mathrm{LiF} \) is largely ionic, while \( \mathrm{LiI} \) has a more balanced sharing of electrons.

Polarizability

Polarizability is a measure of how easily an electron cloud can be distorted by an external electric field. This distortion can induce a dipole in atoms or ions, affecting the nature of bonding. Larger atoms or ions with more loosely held outer electrons are generally more polarizable.
In our examples, iodine is larger than fluorine, leading to higher polarizability. This means that the electron cloud of \( \mathrm{I^-} \) can be more easily distorted by \( \mathrm{Li^+} \). This induced polarization can add a bit of covalent character to the ionic bond in \( \mathrm{LiI} \).
The difference in polarizability between \( \mathrm{F^-} \) and \( \mathrm{I^-} \) has a significant impact on their respective compounds:
- \( \mathrm{F^-} \), being less polarizable, maintains the ionic character of \( \mathrm{LiF} \). - By contrast, the greater polarizability of \( \mathrm{I^-} \) allows \( \mathrm{LiI} \) to exhibit some covalent characteristics.

Covalent Character

Covalent character in a bond occurs when electrons are shared rather than completely transferred between atoms. Although \( \mathrm{LiF} \) and \( \mathrm{LiI} \) are primarily ionic, \( \mathrm{LiI} \) shows more covalent character.
This covalent character emerges mainly due to the polarizability of the iodine ion.When an ion like \( \mathrm{I^-} \) has a highly polarizable electron cloud, its electrons can become shared with the \( \mathrm{Li^+} \) ion, increasing covalent character. The more effectively an electron cloud can align with the presence of other charges, the more the sharing of electrons resembles covalent bonds.
To summarize, the bond in \( \mathrm{LiF} \) is largely ionic, driven by significant differences in electronegativity that result in electron transfer. In contrast, the bond in \( \mathrm{LiI} \) is influenced by the larger polarizability of iodine, leading to partial sharing of electrons and reducing its purely ionic nature.