Concept: Bond strength is influenced by various factors, including bond length, bond order, and the presence of lone pair repulsions. In the case of diatomic molecules, especially those with small atomic sizes and high electronegativity, lone pair-lone pair repulsions can significantly weaken the bond.

Why (A) is correct: Fluorine is a very small atom with high electronegativity. When two fluorine atoms form a single covalent bond (F–F), each fluorine atom has three lone pairs of electrons. Due to the small atomic size, these lone pairs on adjacent fluorine atoms are in close proximity, leading to significant inter-electronic repulsion. This strong repulsion between the non-bonding electron pairs destabilizes the F–F bond, making it unusually weak compared to other halogen-halogen bonds (e.g., Cl–Cl, Br–Br).

Option Analysis:

• A) the repulsion between the non–bonding pairs of electrons of two fluorine atoms is large. This is the correct explanation. The small size of fluorine atoms brings the lone pairs close together, causing strong repulsion and weakening the bond.
• B) the ionization energy of the fluorine atom is very less. Fluorine has a very high ionization energy, not a very less one, due to its small size and high effective nuclear charge. This statement is factually incorrect and irrelevant to bond strength in this context.
• C) the length of the F–F bond is much larger than the bond lengths in other halogen molecules. The F–F bond length (143 pm) is actually the shortest among halogen-halogen bonds (Cl–Cl: 199 pm, Br–Br: 228 pm, I–I: 267 pm). This statement is incorrect.
• D) the F–F bond distance is small and hence the internuclear repulsion between the two F atoms is very less. While the F–F bond distance is small, the internuclear repulsion between the *nuclei* is generally less significant than electron-electron repulsion in determining bond strength for covalent bonds. More importantly, the small bond distance *enhances* the lone pair-lone pair repulsion, which is the primary reason for the weak bond, not less internuclear repulsion.

Correct Answer: (A)