What is hydrogen bonding?

Short Answer

Hydrogen bonding is a special type of strong intermolecular attraction that occurs when hydrogen is bonded to highly electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F). In such molecules, hydrogen becomes slightly positive and is strongly attracted to the negative end of another nearby molecule.

Hydrogen bonds are stronger than dipole-dipole forces but weaker than covalent bonds. They play an important role in determining boiling points, melting points, solubility, and the structure of biological molecules like DNA and proteins.

Detailed Explanation

Hydrogen Bonding

Hydrogen bonding is a strong type of intermolecular force that forms when hydrogen is directly bonded to a small, highly electronegative atom—nitrogen, oxygen, or fluorine. These atoms pull the shared electrons very strongly, leaving hydrogen with a significant partial positive charge (δ+). This positively charged hydrogen is then attracted to the lone pair of electrons on N, O, or F in a neighboring molecule, creating a hydrogen bond.

Hydrogen bonding is stronger than regular dipole-dipole forces because the difference in electronegativity between hydrogen and N, O, or F is very large. However, it is still much weaker than actual chemical bonds like covalent or ionic bonds. Even though a single hydrogen bond is weak, many hydrogen bonds together create a strong effect.

Why Hydrogen Bonding Occurs

For hydrogen bonding to form, three conditions must be satisfied:

  1. Hydrogen must be bonded to N, O, or F.
    These atoms are highly electronegative, pulling electrons strongly.
  2. The molecule must have a lone pair of electrons on N, O, or F.
  3. Nearby molecules must be close enough for attraction to occur.

Because of the large electronegativity difference, hydrogen becomes partially positive and is strongly drawn toward a neighboring atom with a lone pair. This creates a hydrogen bond between two molecules.

Examples of molecules that show hydrogen bonding:

  • Water (H₂O)
  • Ammonia (NH₃)
  • Hydrogen fluoride (HF)
  • Alcohols
  • Carboxylic acids
  • DNA and proteins

Types of Hydrogen Bonding

Hydrogen bonding can occur in two forms:

  1. Intermolecular Hydrogen Bonding

This occurs between molecules.
Example: Water molecules bond with each other through hydrogen bonds.

Effects:

  • High boiling point
  • High surface tension
  • Liquid state at room temperature
  1. Intramolecular Hydrogen Bonding

This occurs within the same molecule.
Example: Ortho-nitrophenol shows intramolecular hydrogen bonding.

Effects:

  • Lower boiling point
  • Less solubility in water
  • Stable molecular shape

Examples of Hydrogen Bonding

Hydrogen bonding is seen in many important substances:

  1. Water (H₂O)

Water is the best example. Each water molecule can form up to four hydrogen bonds.
This gives water:

  • High boiling point (100°C)
  • High specific heat
  • High surface tension
  • Liquid state at room temperature

Without hydrogen bonding, water would be a gas like H₂S.

  1. Ammonia (NH₃)

Ammonia molecules form hydrogen bonds due to the presence of hydrogen attached to nitrogen.
Ammonia has a higher boiling point than expected for its molecular size.

  1. Hydrogen Fluoride (HF)

HF has very strong hydrogen bonds because fluorine is the most electronegative element.

  1. Alcohols

Ethanol and methanol show hydrogen bonding, which makes them dissolve well in water.

  1. DNA and Proteins

In DNA, hydrogen bonds hold the two strands together (A–T and G–C bonding).
Proteins maintain their shapes through hydrogen bonds.

Importance of Hydrogen Bonding

Hydrogen bonding affects the physical and chemical properties of many substances.

  1. High Boiling and Melting Points

Hydrogen-bonded compounds require more energy to separate molecules, so their boiling and melting points are high.

  1. Solubility

Hydrogen bonding helps polar substances dissolve in water.
Example: sugar dissolves easily due to hydrogen bonding with water.

  1. Surface Tension

Water’s high surface tension comes from strong hydrogen bonds.

  1. Biological Structures

Hydrogen bonds stabilize:

  • DNA double helix
  • Protein folding
  • Enzyme-substrate interactions
  1. Ice Structure

Ice has an open, hexagonal structure due to hydrogen bonding, causing ice to float on water.

  1. Liquid Water Properties

Hydrogen bonding explains:

  • High heat of vaporization
  • High heat capacity
  • Unusual density behavior
Conclusion

Hydrogen bonding is a strong intermolecular force that occurs when hydrogen is bonded to nitrogen, oxygen, or fluorine. It plays an important role in determining the physical and chemical properties of substances. Hydrogen bonding influences boiling points, melting points, solubility, and biological functions. It is essential in water behavior, ice formation, and the stability of DNA and proteins. This unique force helps explain many important characteristics of matter in chemistry and biology.