Water (H₂O) is essential for life on Earth. Its ability to sustain biological processes and its distinctive physical and chemical properties, such as remaining liquid over a wide temperature range and dissolving many substances, depend entirely on the specific chemical bonds it forms. Understanding the nature of these bonds explains water’s unique behavior.
The Covalent Bond Inside the Water Molecule
The atoms within a single water molecule are held together by a strong connection known as a polar covalent bond. This type of bond involves the sharing of electrons between the single oxygen atom and each of the two hydrogen atoms. However, the sharing of these electrons is not equal, which is a characteristic of polarity.
The oxygen atom has a much greater tendency to attract electrons compared to the hydrogen atoms, a property known as high electronegativity. Because of this difference, the shared electrons spend significantly more time orbiting the oxygen nucleus than the hydrogen nuclei. This unequal distribution of negative charge causes the oxygen end of the molecule to develop a partial negative charge (\(\delta^-\)), while the hydrogen ends acquire a partial positive charge (\(\delta^+\)).
The resulting charge separation makes the water molecule a dipole, meaning it has two distinct electric poles. The molecule also has a non-linear, or bent, shape, which prevents the partial charges from canceling out. This ensures the molecule remains polar, which is the foundation for how water molecules interact with each other and with other substances.
How Water Molecules Connect Through Hydrogen Bonds
The polarity established by the internal covalent bonds leads directly to the formation of a secondary, weaker attraction between separate water molecules, called a hydrogen bond. This attraction is a form of intermolecular force, meaning it occurs between molecules, not within them. A hydrogen bond forms when the partially positive hydrogen atom of one water molecule is electrostatically attracted to the partially negative oxygen atom of a neighboring water molecule.
Each water molecule can participate in up to four hydrogen bonds with its neighbors, creating an extensive, dynamic network in liquid water. This network holds the liquid together and gives water stability. The hydrogen bond is significantly weaker than the polar covalent bond holding the atoms together within a single molecule (467 kilojoules per mole versus 4 to 40 kilojoules per mole for a hydrogen bond).
Why Water’s Bonds Matter for Life
The combination of strong polar covalent bonds and weaker hydrogen bonds gives water properties essential for life. One property is water’s high specific heat, the amount of energy required to raise its temperature. When heat is added, much of the energy is used to break the hydrogen bond network rather than increasing molecular speed. This allows large bodies of water to absorb and release significant heat with only slight temperature changes, moderating global and cellular temperatures.
Another property is water’s ability to dissolve many substances, earning it the nickname “universal solvent”. The molecule’s polarity allows it to surround and pull apart other polar molecules and ionic compounds. For instance, the partially negative oxygen atom attracts the positive ions of a salt, while the partially positive hydrogen atoms attract the negative ions, effectively dissolving the compound.
Extensive hydrogen bonding also drives water’s cohesive and adhesive properties, which are important for plant life. Cohesion is the attraction of water molecules to one another, creating surface tension and allowing water to form droplets. Adhesion is the attraction of water molecules to other surfaces. These two forces enable water to move upward through the narrow vessels of plants—a process called capillary action—transporting nutrients from the roots to the leaves.