What is a Hydrogen Bond?
We’ve learned about covalent and ionic bonds, but what about hydrogen bonds? As you may have guessed from the name, hydrogen bonds can only be formed by molecules containing at least one hydrogen atom. Unlike covalent or ionic bonds, hydrogen bonds aren’t formed by sharing or exchanging electrons but rather by something called polarity.
Some atoms “want” electrons more strongly than others, because they are closer to having their outer electron shell filled. These atoms are said to have a higher electronegativity value, meaning that they attract electrons very strongly. When covalent bonds form between atoms with different electronegativity values, the shared electrons will spend more time near the nucleus of the more electronegative atom because it attracts them more strongly.
For example, a water molecule is composed of two hydrogen atoms and an oxygen atom. Oxygen is more electronegative than hydrogen, meaning it “wants” electrons more strongly. Due to this attraction, the orbiting electrons in a water molecule spend more time near the oxygen atom than either of the hydrogen atoms.
This uneven dispersal of electrons gives the oxygen end of the molecule a slight negative charge and the hydrogen ends of the molecule slightly positive charges. Water and other molecules with unevenly shared electrons are called “polar” molecules.
Hydrogen bonds form when the slightly negative end of one polar molecule is attracted to the slightly positive end of another polar molecule. Going with our water example, if another water molecule were to come along, the oxygen atom of one molecule would be attracted to the hydrogen atom of the other molecule. The attractive force between the two oppositely charged atoms is a hydrogen bond.
Most commonly, hydrogen bonds involve one or more hydrogen atoms covalently bonded with oxygen, fluorine, or nitrogen. They can only form between polar molecules.
It takes significantly less energy to break a hydrogen bond than to break a covalent or ionic bond. In fact, the hydrogen bond between an oppositely charged oxygen atom and hydrogen atom from different molecules has only about 1/20th the strength of the covalent bond between an oxygen atom and hydrogen atom in the same molecule.
Despite their lack of strength, hydrogen bonds are very important to organic life. Hydrogen bonds are what give water its unique, life-sustaining properties. Hydrogen bonds also hold the strands of our DNA together. Without hydrogen bonds, life as we know it would not be able to exist.
User Qwerter (artist). (2011). Hydrogen Bonds [digital art]. Retrieved from https://commons.wikimedia.org/wiki/File:3D_model_hydrogen_bonds_in_water.svg