Question

Which of the following shows maximum hydrogen bonding?

• A. HF
• B. HCl
• C. HBr
• D. HI

Hint:

Among molecules in the same family, an unexpectedly high boiling point despite a low molecular mass is usually a sign of strong hydrogen bonding. Check which hydrogen halide breaks the expected mass-based trend most sharply, and recall which atoms must be attached to hydrogen for such bonding to occur.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Hydrogen bonding is a special type of dipole-dipole attraction between a hydrogen atom covalently bonded to a highly electronegative atom (F, O, or N) and another electronegative atom with a lone pair of electrons.

Step 2: Detailed Explanation:
The strength of a hydrogen bond depends on the electronegativity of the atom attached to hydrogen.
• Fluorine (F) is the most electronegative element in the periodic table.
• In the series HF, HCl, HBr, and HI, only HF is capable of forming strong intermolecular hydrogen bonds because the electronegativity of Cl, Br, and I is not high enough to create the necessary polarity.
• Due to this strong hydrogen bonding, HF has an unexpectedly high boiling point compared to other hydrogen halides.

Step 3: Final Answer:
HF (Hydrogen Fluoride) shows the maximum hydrogen bonding.

Answer 2

Concept:

• Hydrogen bonding occurs when hydrogen is attached to a small, highly electronegative atom (fluorine, oxygen or nitrogen) that also carries a lone pair to attract hydrogen from a neighbouring molecule.
• A telltale sign of hydrogen bonding in a series of similar molecules is an abnormally high boiling point compared to what molecular mass alone would predict.
• Comparing actual boiling points, instead of only comparing electronegativity values, gives direct physical evidence for identifying the strongest hydrogen bonding.

Step 1: Note the trend expected from molecular mass alone.
Going from HCl to HBr to HI, molecular mass increases steadily, so van der Waals forces should increase and the boiling point should rise smoothly in that order: HCl < HBr < HI (roughly $-85^\circ C$, $-67^\circ C$, $-35^\circ C$).

Step 2: Compare this with the actual boiling point of HF.
Although HF has the smallest molecular mass of the four, it actually boils near $19$-$20^\circ C$, far higher than any of the heavier hydrogen halides listed above — a large positive deviation from the mass-based trend.

Step 3: Explain the deviation and identify the answer.
An unexpectedly high boiling point for the lightest molecule means extra energy is needed to pull HF molecules apart, which happens only if an attractive force stronger than plain van der Waals forces exists between them. That extra force is hydrogen bonding, made possible because fluorine is small and extremely electronegative. Among the four options, only HF shows this anomaly.

Final Answer: HF shows the maximum hydrogen bonding.