2.4.1—Hydrogen bonding between water molecules

Water is a covalent but polar molecule. Oxygen attracts the shared electrons more strongly than hydrogen, so the oxygen end is slightly negative (δ−) and the hydrogen ends are slightly positive (δ+), while the whole molecule remains electrically neutral.

• Within one molecule: Covalent bonds share electrons between oxygen and hydrogen.
• Between molecules: The δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighbouring molecule; this intermolecular attraction is a hydrogen bond.
• Network effect: Hydrogen bonds continually break and reform, but many together give water a cohesive network.
• Property link: This network helps explain water’s solvent action, cohesion and surface tension, relatively high specific heat capacity, relatively high latent heat of vaporisation, and why solid water is less dense than liquid water.

Unequal electron sharing creates polarity; polarity creates attractions between neighbouring molecules; the resulting hydrogen-bond network gives water properties that are important in living systems. The property is therefore a consequence of intermolecular attraction, not of replacing the covalent bonds inside each molecule.

Do not draw full ionic charges on water, call the molecule an ion, or place a hydrogen bond inside one molecule. A hydrogen bond is the attraction between the δ+ hydrogen of one molecule and the δ− oxygen of another.