Water (H₂O) is a simple molecule composed of two hydrogen atoms bonded to a single oxygen atom. Water’s unique behavior, which makes it pervasive on Earth and fundamental to life, is driven by the forces of attraction that exist between neighboring water molecules. This powerful intermolecular attraction, known as the hydrogen bond, governs everything from how water flows to how it freezes.
Understanding Water Polarity
Water’s unique properties begin with the uneven distribution of electrical charge within the molecule. Oxygen is significantly more electronegative than hydrogen, meaning it exerts a stronger pull on the shared electrons in the covalent bonds. This unequal sharing causes the oxygen atom to acquire a slight negative charge (\(\delta^-\)), while each hydrogen atom develops a slight positive charge (\(\delta^+\)).
The bent shape of the water molecule, along with this charge imbalance, ensures that the molecule as a whole is polar. A polar molecule has a net dipole moment, with one end being slightly positive and the other end slightly negative, much like a tiny magnet. This permanent separation of charge is the prerequisite for water’s ability to form its characteristic intermolecular bonds.
The Mechanism of Hydrogen Bond Formation
A hydrogen bond is an attraction that occurs between two molecules, forming when the partial positive hydrogen atom of one molecule is drawn toward the partial negative oxygen atom of a neighbor. This attraction is distinct from the stronger covalent bonds that hold the atoms within a single water molecule.
The bond is an electrostatic attraction, much weaker than a covalent bond—only about a twentieth of the strength of the internal covalent oxygen-hydrogen bond—but strong enough to create a highly organized network among water molecules.
In liquid water, this network is highly dynamic, with hydrogen bonds constantly breaking and reforming at a rapid pace. However, at any given moment, a significant portion of the molecules are linked together. Each water molecule has the potential to form up to four hydrogen bonds with its neighbors, arranged in a tetrahedral structure, which creates a structured, three-dimensional arrangement.
Unique Physical Properties Caused by Bonding
The extensive network of hydrogen bonds leads directly to physical properties that are anomalous compared to most other liquids. These properties allow water to support life and shape Earth’s climate.
The attraction between water molecules leads to strong cohesion, the tendency of like molecules to stick together. This cohesive force, driven by hydrogen bonds, is responsible for water’s high surface tension, allowing small insects to walk across its surface. Water also exhibits adhesion, the attraction to other polar substances, enabling capillary action where water is drawn upward against gravity, such as in plant vascular systems.
Water possesses a high specific heat capacity, meaning it absorbs or releases large amounts of heat energy with minimal temperature change. This occurs because incoming heat must first break the extensive network of hydrogen bonds before increasing the molecules’ kinetic energy. This property allows large bodies of water, like oceans, to moderate global temperatures and helps organisms maintain a stable internal temperature.
Another unusual consequence of hydrogen bonding is the density anomaly of water, where the solid state (ice) is less dense than the liquid state. As water cools below 4 degrees Celsius, the hydrogen bonds force the molecules into a more orderly, open, crystalline lattice structure, which is less compact than the randomly associated molecules in liquid water. This open structure causes ice to float on liquid water, insulating the water beneath and protecting aquatic life during cold conditions.