Hydrogen Bonding in Water Dimer

Steps:

• Each water molecule (H₂O) has two hydrogen atoms and two lone pairs on oxygen, enabling hydrogen bonds.
• A hydrogen bond forms when the H of one water aligns with a lone pair on another's oxygen.
• For two isolated molecules, the stable configuration positions one H from the donor to one lone pair on the acceptor.
• Steric and angular constraints (tetrahedral geometry, ~104.5° bond angle) prevent more than one bond without instability.

Why A is correct:

• The water dimer structure, confirmed by spectroscopy, forms a single linear O-H···O hydrogen bond as the maximum stable interaction.

Why the others are wrong:

• B: Two bonds would require a bifurcated donor (both H to one O), but geometry distorts angles beyond feasible limits.
• C: Three bonds exceed available donor-acceptor pairs without overlapping sites.
• D: Four bonds imply full saturation like in ice, requiring multiple neighbors, not possible for just two molecules.

Final answer: A