Biology · Unit 1: Chemistry of Life · 14 min read · Updated 2026-05-10
Structure of Water and Hydrogen Bonding — AP Biology
AP Biology · Unit 1: Chemistry of Life · 14 min read
1. Polar Covalent Structure of the Water Molecule★★☆☆☆⏱ 4 min
A single water molecule has molecular formula $H_2O$, with one central oxygen atom covalently bonded to two hydrogen atoms. Oxygen has a Pauling electronegativity of ~3.5, while hydrogen has an electronegativity of ~2.1.
\Delta EN = 3.5 - 2.1 = 1.4
Oxygen pulls shared electrons closer, creating a partial negative charge ($\delta^-$) on oxygen and partial positive charges ($\delta^+$) on each hydrogen. Water has a bent geometry (104.5° bond angle) from oxygen's two lone electron pairs, so partial charges do not cancel, making water a permanent dipole. If water were linear, dipoles would cancel and water would be nonpolar.
Exam tip: When asked to justify a claim about molecular polarity, always address both bond polarity and molecular geometry; AP exam graders require both for full credit.
2. Intermolecular Hydrogen Bonding Between Water Molecules★★☆☆☆⏱ 3 min
In liquid water, each water molecule can form up to four hydrogen bonds: the two partially positive H atoms each donate a hydrogen bond to the $\delta^-$ oxygen of a neighboring water, and the two lone pairs on the central oxygen each accept a hydrogen bond from a $\delta^+$ H on two other neighbors. Individual hydrogen bonds are weak (~20 kJ/mol vs ~460 kJ/mol for an O-H covalent bond), but they are strong collectively because billions form simultaneously in liquid water. They break and re-form constantly, but the network is always maintained.
Exam tip: Always explicitly distinguish between intramolecular covalent bonds within a water molecule and intermolecular hydrogen bonds between different water molecules; this is a very common AP exam point of confusion.
3. Emergent Biological Properties of Water★★★☆☆⏱ 5 min
**Cohesion and adhesion**: Cohesion is attraction between water molecules due to hydrogen bonding, creating high surface tension. Adhesion is attraction between water and other polar/charged molecules, enabling capillary action.
**High specific heat capacity**: Energy is required to break hydrogen bonds before temperature can increase, so water absorbs large amounts of heat before warming, stabilizing organism and ecosystem temperatures.
**High heat of vaporization**: A water molecule must break all its hydrogen bonds to evaporate, so evaporation absorbs large amounts of heat, enabling effective evaporative cooling.
**Versatility as a solvent**: Water forms hydration shells around polar/charged (hydrophilic) molecules, enabling biological reactions. Nonpolar (hydrophobic) molecules aggregate, driving formation of cell membranes and protein tertiary structure.
Exam tip: When asked to connect water’s properties to a biological scenario, always explicitly link the observed property back to hydrogen bonding; you will not earn full credit without this causal connection.
Common Pitfalls
Why: Students confuse intramolecular bonds that make up a single water molecule with intermolecular attractions between different water molecules
Why: Students confuse the collective strength of many hydrogen bonds with the strength of a single hydrogen bond
Why: Students forget that molecular geometry, not just bond polarity, determines net molecular polarity
Why: Students memorize the list of properties but forget the causal link that AP exam questions require for justification points
Why: Students confuse "uncharged" with "polar"; nonpolar molecules have no partial charges to interact with water’s dipole