1.1 Structure of Water and Hydrogen Bonding - AP Biology (Updated 2025-2026)

Structure of Water and Hydrogen Bonding

Water’s Molecular Structure

• Water (H₂O) consists of two hydrogen atoms bonded to one oxygen atom.
• The bonds within a water molecule are polar covalent bonds: electrons are shared unequally and are pulled more strongly toward oxygen.
• As a result, the oxygen end has a slight negative charge, while the hydrogen ends have slight positive charges. Water is therefore a polar molecule.
• The oppositely charged ends of neighboring water molecules attract, forming hydrogen bonds.
• Hydrogen bonds are attractions between molecules. They are different from the covalent bonds that hold the atoms within a single water molecule together, and they continually form and break.

Properties Caused by Hydrogen Bonding

• High specific heat capacity: Water takes a relatively large amount of energy to increase in temperature. Hydrogen-bond attractions resist the increased molecular motion associated with heating. This helps water-rich organisms and environments resist rapid temperature changes.
• High heat of vaporization: A substantial amount of energy is needed for water to change from liquid to gas because molecules must overcome their attractions to neighboring molecules.
  • Evaporative cooling: When sweat evaporates, it absorbs heat from the skin, helping cool the body. Dogs also cool themselves through evaporation when panting.
• Cohesion: Water molecules attract and stick to one another. This helps water form droplets and contributes to the number of drops that can gather on a penny.
• Surface tension: Cohesion makes the water’s surface difficult to break, allowing some organisms, such as water striders, to move across it.
• Adhesion: Water also sticks to other polar surfaces. In plants, its attraction to the polar inner walls of xylem tubes helps water move through narrow passages. Together with cohesion, adhesion contributes to capillary action, which helps water travel upward in plants.

Overall Lesson

Water’s polarity leads to hydrogen bonding, and hydrogen bonding explains many of water’s important physical properties. These properties support temperature regulation, evaporative cooling, and water movement in plants.

Speaker or Source

Mr. Poser, the AP Biology teacher and presenter.