Each water molecule can form hydrogen bonds with neighbouring water molecules. What is the maximum number of hydrogen bonds that a single H2O molecule can form?

Introduction / Context:
Hydrogen bonding in water explains many of its unusual properties, such as high boiling point, high surface tension and the open structure of ice. In a three dimensional network, each water molecule can both donate and accept hydrogen bonds. This question asks for the maximum number of hydrogen bonds a single water molecule can form at one time with its neighbours.

Given Data / Assumptions:

• A water molecule has the formula H2O.
• Oxygen has two lone pairs of electrons in water.
• Each hydrogen attached to oxygen can participate in hydrogen bonding.
• We assume an ideal hydrogen bonding network such as in ice.

Concept / Approach:
Hydrogen bonding requires a hydrogen atom covalently bonded to a highly electronegative atom (such as oxygen, nitrogen or fluorine) and a lone pair on another such electronegative atom. In water, each molecule has two hydrogen atoms that can donate hydrogen bonds and two lone pairs on oxygen that can accept hydrogen bonds. Therefore, a single water molecule can form up to two hydrogen bonds as a donor and two as an acceptor, giving a maximum of four hydrogen bonds in a tetrahedral arrangement.

Step-by-Step Solution:
Step 1: Draw or imagine the Lewis structure of water, with oxygen in the centre, two O–H bonds and two lone pairs on oxygen.Step 2: Each O–H bond can donate a hydrogen bond to the lone pair of an oxygen atom on a neighbouring water molecule.Step 3: Each lone pair on the central oxygen can accept a hydrogen bond from a hydrogen atom on neighbouring water molecules.Step 4: There are two hydrogens and two lone pairs, so water can donate two and accept two hydrogen bonds.Step 5: Add these to find the maximum total: 2 donor bonds + 2 acceptor bonds = 4 hydrogen bonds.Step 6: This matches the tetrahedral coordination seen in ice, where each water molecule is surrounded by four neighbours.

Verification / Alternative check:
In solid ice, crystallographic studies show that each water molecule is tetrahedrally coordinated to four others via hydrogen bonds. In liquid water, the network is more dynamic but still tends to approach four hydrogen bonded neighbours on average. Textbook diagrams of hydrogen bonding in water emphasise this fourfold pattern, reinforcing the conclusion that the maximum number is four.

Why Other Options Are Wrong:
Option a, one hydrogen bond, ignores the fact that water has two hydrogens and two lone pairs. Option b, three hydrogen bonds, underestimates the capacity of the two lone pairs and two hydrogens. Option d, two hydrogen bonds, would be correct for molecules that can act only as donors or only as acceptors, but water can do both. Option e, five, exceeds the number of hydrogen bond donors and acceptors available on a single water molecule.

Common Pitfalls:
A frequent misunderstanding is to think that only the hydrogens can form hydrogen bonds and forget the role of lone pairs, leading to the answer two. Another pitfall is to count all possible near neighbours in the liquid rather than the maximum bonding sites on a single molecule. To avoid these mistakes, focus on the structure of water: two O–H bonds and two lone pairs translate directly into four possible hydrogen bonds.

Final Answer:
The maximum number of hydrogen bonds that one water molecule can form is 4, two as a donor and two as an acceptor.