Which of the following molecules is held together by hydrogen bond?

Analysis

The question asks us to identify which of the given molecules exhibits hydrogen bonding. Hydrogen bonding is a special, strong type of dipole-dipole attraction that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom (specifically Fluorine (F), Oxygen (O), or Nitrogen (N)).

Concept Explanation

For a molecule to form a hydrogen bond, two conditions must be met:

1. It must contain a hydrogen atom directly bonded to a highly electronegative atom (F, O, or N).
2. The electronegative atom must have at least one lone pair of electrons to interact with the hydrogen of a neighboring molecule.

Because F, O, and N have very high electronegativity, they pull the shared electrons in the covalent bond strongly toward themselves, creating a significant partial positive charge ($\delta+$) on the hydrogen atom and a partial negative charge ($\delta-$) on the electronegative atom. This strong polarity allows the hydrogen of one molecule to be attracted to the lone pair of the electronegative atom in another molecule.

Step-by-Step Solution

Let's evaluate the options:

• A. $CH_4$ (Methane): Carbon is not sufficiently electronegative to create the strong dipole required for hydrogen bonding. $CH_4$ is a non-polar molecule held together by weak Van der Waals forces.
• B. $HBr$ (Hydrogen bromide): While bromine is electronegative, it is not electronegative enough to support hydrogen bonding. Hydrogen bonding is generally restricted to F, O, and N.
• C. $H_2SO_4$ (Sulfuric acid): While this molecule contains oxygen, the hydrogen atoms are bonded to oxygen atoms. However, in the context of simple molecular comparisons in JAMB, we look for the classic example of hydrogen bonding.
• D. $HF$ (Hydrogen fluoride): Fluorine is the most electronegative element in the periodic table. When hydrogen is bonded to fluorine, the bond is extremely polar. This allows $HF$ molecules to form very strong hydrogen bonds with each other.

Comparing these, $HF$ is the textbook example of a molecule held together by hydrogen bonding.

Conclusion

$HF$ exhibits the strongest hydrogen bonding among the choices provided because fluorine has the highest electronegativity of the atoms involved.

Exam Tip

Remember the acronym "FON" (Fluorine, Oxygen, Nitrogen). If you see a molecule containing Hydrogen bonded to any of these three, it is highly likely to exhibit hydrogen bonding!