Concept: Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom (like F, O, or N) and is attracted to another electronegative atom in a different molecule or the same molecule.

The strength and extent of hydrogen bonding depend on the electronegativity of the atom bonded to hydrogen and the number of lone pairs available for interaction.

Why (A) H₂O is correct:

Water (H₂O) has two hydrogen atoms and two lone pairs on the oxygen atom. This allows each water molecule to form, on average, four hydrogen bonds with other water molecules (two through its hydrogen atoms and two through its lone pairs on oxygen). This extensive network of hydrogen bonding is responsible for many of water's unique properties, such as its high boiling point and specific heat capacity.

Option Analysis:

• A) H₂O: Oxygen is highly electronegative, and each H₂O molecule can form up to four hydrogen bonds (two as donor, two as acceptor). This leads to the maximum number of H-bonds among the given options.
• B) H₂Se: Selenium is much less electronegative than oxygen. Therefore, H₂Se forms very weak or negligible hydrogen bonds.
• C) H₂S: Sulfur is less electronegative than oxygen. H₂S forms very weak or negligible hydrogen bonds compared to H₂O.
• D) HF: Fluorine is the most electronegative element. While HF forms strong hydrogen bonds, each HF molecule can only form two hydrogen bonds (one as a donor, one as an acceptor) due to having only one hydrogen atom and three lone pairs. This is less than the four hydrogen bonds formed by H₂O on average.

Correct Answer: (A)