A hydrogen bond in water is a weak intermolecular attraction between the partially positive hydrogen atom of one water molecule and the partially negative oxygen atom of another. This concept is required for both SL and HL in IB Biology subtopic A1.1 Water.
How Hydrogen Bonds Form
- Within a water molecule, oxygen and hydrogen are joined by polar covalent bonds. The electrons are shared, but not equally.
- Oxygen attracts the shared electrons more strongly because it is more electronegative. Oxygen therefore develops a partial negative charge (), while each hydrogen develops a partial positive charge ().
- The hydrogen of one water molecule is attracted to the oxygen of a neighbouring molecule. This attraction is the hydrogen bond.
A hydrogen bond is weaker than the covalent bonds within a water molecule. However, large numbers of hydrogen bonds collectively produce important properties of water.
| Bond or attraction | Location and significance |
|---|---|
| Polar covalent bond | Occurs between oxygen and hydrogen within one water molecule; electrons are shared unequally. |
| Hydrogen bond | Occurs between neighbouring water molecules; it is an attraction between partial charges. |
Hydrogen bonding causes cohesion, meaning that water molecules attract one another. Cohesion contributes to surface tension and helps maintain continuous columns of water under tension in xylem vessels. Hydrogen bonds continually break and reform because water molecules are in constant motion.
A common misconception is that the hydrogen bond joins hydrogen and oxygen within one water molecule. That bond is covalent; the hydrogen bond usually shown in IB water diagrams is the attraction between separate water molecules.
Exam Technique
For an explain question, link the ideas in sequence: unequal electron sharing → partial charges → attraction between neighbouring molecules → hydrogen bonding. Label and accurately, and do not describe hydrogen bonds as strong covalent bonds.
