Structure of Water and Hydrogen Bonding

Definition

A hydrogen bond is an attraction between a hydrogen atom carrying a partial positive charge and a nearby electronegative atom carrying a partial negative charge. In liquid water, hydrogen bonds continually break and form between different molecules.

Why Polarity Matters

Oxygen attracts shared electrons more strongly than hydrogen. The O–H bonds are polar, and because the molecule is bent, the bond dipoles do not cancel. Water therefore has a partially negative oxygen region and partially positive hydrogen regions.

From Molecular Attraction to Biological Properties

• Cohesion: water molecules attract one another.
• Adhesion: water interacts with other polar surfaces.
• High specific heat: added energy first disrupts intermolecular attractions, limiting rapid temperature change.
• Evaporative cooling: higher-energy molecules escape, lowering the average kinetic energy of the liquid that remains.

Worked Example

Question: Why can sweating cool the skin even when the liquid sweat begins at body temperature?

Reasoning: The fastest water molecules are most likely to escape into the gas phase. Breaking their attractions to neighboring molecules requires energy, and the escaping molecules carry energy away from the skin.

Answer: Evaporation removes relatively high-energy molecules, so the remaining liquid and skin have a lower average thermal energy.

Common Mistakes

• Calling a hydrogen bond an O–H covalent bond inside one water molecule.
• Claiming water molecules are charged ions; water is polar but has no net charge.
• Explaining cohesion without identifying attractions between separate molecules.

Key Takeaways

• Bond polarity plus bent geometry gives water a molecular dipole.
• Hydrogen bonds are weaker than covalent bonds but collectively shape bulk behavior.
• Biological effects should be connected from molecular interaction to observable property.