A hydrogen bond is a distinct type of attractive force that occurs between molecules. It represents a special interaction where a hydrogen atom, already connected to one molecule, forms an attraction with an electronegative atom from a different molecule. This molecular attraction is weaker than the bonds holding atoms together within a single molecule, yet its widespread presence contributes significantly to the properties of many substances.
How Hydrogen Bonds Form
Hydrogen bonds arise from the unequal sharing of electrons within a molecule, known as polarity. When a hydrogen atom forms a covalent bond with a highly electronegative atom, such as oxygen, nitrogen, or fluorine, the electronegative atom pulls the shared electrons closer to itself. This electron-pulling creates a slight negative charge on the electronegative atom and leaves the hydrogen atom with a slight positive charge. For example, in a water molecule, oxygen is highly electronegative and draws electrons away from the hydrogen atoms.
The resulting partial positive charge on the hydrogen atom is then attracted to a partial negative charge on another electronegative atom from a neighboring molecule. This electrostatic attraction forms the hydrogen bond. These interactions are strongest when the hydrogen atom is bonded to oxygen, nitrogen, or fluorine due to their high electronegativity.
Hydrogen Bonds and Water’s Unique Properties
Hydrogen bonds are responsible for many of water’s distinctive properties, making it an exceptional substance on Earth. One notable effect is water’s relatively high boiling point of 100°C, which is unusually elevated for a molecule of its small size. A substantial amount of energy is required to break the extensive network of hydrogen bonds holding water molecules together in the liquid state, allowing them to escape as gas.
Water also possesses a high specific heat capacity, meaning it can absorb a large amount of heat without a significant rise in temperature. The energy absorbed goes into breaking hydrogen bonds before increasing the kinetic energy of the molecules, which helps moderate global temperatures. Surface tension, which allows water to resist external forces and form droplets, is a direct consequence of strong cohesive hydrogen bonds at the liquid’s surface. Ice floats on liquid water because hydrogen bonds arrange water molecules into a more open, crystal lattice structure upon freezing, making ice about 9% less dense than liquid water at 0°C.
Hydrogen Bonds in Biological Structures
Hydrogen bonds play an important role in shaping and stabilizing the structures of biological molecules, particularly DNA and proteins. In DNA, hydrogen bonds connect the two long strands of the double helix, forming the “rungs” of the twisted ladder structure. Adenine (A) always pairs with thymine (T) via two hydrogen bonds, while guanine (G) pairs with cytosine (C) through three hydrogen bonds.
These specific pairings ensure the accurate replication and transmission of genetic information. Similarly, in proteins, hydrogen bonds between atoms in the polypeptide backbone contribute to the formation of specific three-dimensional shapes, known as secondary structures. Common secondary structures, such as alpha-helices and beta-sheets, are stabilized by these recurring hydrogen bond patterns, which are important for a protein’s biological function.
How Hydrogen Bonds Differ from Other Chemical Bonds
Hydrogen bonds are distinct from other common chemical bonds, such as covalent and ionic bonds, primarily in their nature and strength. Covalent bonds involve the sharing of electron pairs between atoms to form molecules, while ionic bonds result from the complete transfer of electrons between atoms, creating oppositely charged ions that attract each other. Both covalent and ionic bonds are intramolecular, meaning they occur within a single molecule or compound, holding the atoms together.
In contrast, hydrogen bonds are intermolecular forces, existing between separate molecules or different parts of the same large molecule. Individually, a hydrogen bond is significantly weaker than a typical covalent or ionic bond, often being about 5-10% of their strength, with energies ranging from approximately 4 to 50 kJ/mol. However, their collective presence in large numbers provides significant stability and influence over the physical and biological properties of substances.