Concept: Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom (F, O, or N). The strength of the hydrogen bond depends on the electronegativity of the atoms involved and the size of the atoms.

Why (C) is correct:

The strength of a hydrogen bond increases with the electronegativity of the atom to which hydrogen is directly bonded and the electronegativity of the atom acting as the hydrogen bond acceptor. Fluorine (F) is the most electronegative element (Pauling scale: 3.98), followed by Oxygen (O) (Pauling scale: 3.44), and then Sulfur (S) (Pauling scale: 2.58).

In option (C), F – H – – – F, both the donor atom (F) and the acceptor atom (F) are highly electronegative. This leads to a very strong polarization of the H-F bond, making the hydrogen atom highly positive and the fluorine atom highly negative, resulting in the strongest hydrogen bond among the given options.

Option Analysis:

• A) O – H – – – S: Oxygen is electronegative, but sulfur is less electronegative than oxygen, making this hydrogen bond weaker than those involving only O or F.
• B) S – H – – – O: Sulfur is less electronegative than oxygen, so the S-H bond is less polarized than O-H or F-H, resulting in a weaker hydrogen bond.
• D) O – H – – – O: This is a strong hydrogen bond, as both oxygen atoms are highly electronegative. However, fluorine is more electronegative than oxygen, making the F-H...F bond stronger than the O-H...O bond.

Correct Answer: (C)