How many hydrogen bonds can a single water molecule form? How many does it typically form and why? Draw a diagram that shows how water molecules interact to form hydrogen bonds.
The correct answer and explanation is :
A single water molecule can form up to 4 hydrogen bonds: it can donate 2 hydrogen bonds (via its two hydrogen atoms) and accept 2 hydrogen bonds (via the two lone pairs on its oxygen atom).
Typically, in liquid water, each molecule forms about 3.4 hydrogen bonds on average, while in ice, each water molecule forms exactly 4 hydrogen bonds, creating a tetrahedral structure.
Explanation (≈300 words):
Water (H₂O) is a polar molecule, with the oxygen atom being more electronegative than hydrogen. This causes the oxygen to carry a partial negative charge (δ⁻), while the hydrogen atoms carry partial positive charges (δ⁺). As a result, the hydrogen of one water molecule is attracted to the lone pair electrons on the oxygen of another molecule—this electrostatic attraction is called a hydrogen bond.
Each water molecule has:
Two hydrogen atoms (each can form a hydrogen bond as a donor),
Two lone pairs on oxygen (each can accept a hydrogen bond from another water molecule).
Therefore, the maximum number of hydrogen bonds per water molecule is four: two donated, two accepted.
In ice, where water is in a solid crystalline state, molecules are arranged in a regular hexagonal lattice, maintaining exactly four hydrogen bonds per molecule. This open structure accounts for ice’s lower density compared to liquid water.
In liquid water, thermal motion disrupts the ideal hydrogen bonding, and the structure becomes more dynamic. This leads to an average of 3.4 hydrogen bonds per molecule—some molecules temporarily form fewer than four bonds.
This hydrogen bonding behavior is key to water’s unique properties such as:
High boiling point for a small molecule,
High surface tension,
Density anomaly (ice floats on water),
Excellent solvent properties for polar and ionic compounds.
