What is hydrogen bonding?

Short Answer

Hydrogen bonding is a special type of intermolecular attraction that occurs when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. In such molecules, hydrogen develops a partial positive charge and is strongly attracted to the lone pair of nearby electronegative atoms.

Hydrogen bonds are weaker than covalent bonds but stronger than most other intermolecular forces. They are responsible for many unique properties of water, such as its high boiling point, surface tension, and ability to dissolve many substances.

Detailed Explanation :

Hydrogen Bonding

Hydrogen bonding is an important concept in chemistry that explains many physical and chemical properties of substances, especially water and biological molecules. It is not a chemical bond like ionic or covalent bonds; instead, it is a strong form of intermolecular force. Hydrogen bonds arise from the attraction between a hydrogen atom attached to an electronegative atom and another electronegative atom nearby.

This force of attraction plays a major role in determining the structure, behaviour, and interactions of molecules in liquids, solids, and even in living organisms. Without hydrogen bonding, many essential processes such as DNA stability, protein folding, and the unique behaviour of water would not occur.

Meaning of Hydrogen Bonding

Hydrogen bonding can be defined as:

“A strong intermolecular attraction between a hydrogen atom attached to an electronegative atom (such as O, N, or F) and another electronegative atom having a lone pair of electrons.”

This occurs only when hydrogen is bonded to atoms with high electronegativity and small size.

Conditions Required for Hydrogen Bonding

Hydrogen bonding occurs only when three conditions are met:

  1. Presence of hydrogen

Hydrogen must be covalently bonded to an electronegative element.

  1. High electronegativity

Atoms like oxygen (O), nitrogen (N), and fluorine (F) strongly attract electrons and create a partial negative charge.

  1. Small atomic size

Smaller atoms allow stronger attraction between hydrogen and lone pairs.

Because fluorine, oxygen, and nitrogen meet these conditions, hydrogen bonds commonly occur in:

  • H–F
  • H–O
  • H–N

How Hydrogen Bonding Occurs

When hydrogen is bonded to an electronegative atom:

  1. The electronegative atom pulls the shared electrons closer.
  2. Hydrogen becomes partially positive (δ⁺).
  3. The electronegative atom becomes partially negative (δ⁻).
  4. The δ⁺ hydrogen is attracted to a δ⁻ atom of a neighbouring molecule.
  5. This attraction forms a hydrogen bond.

This attraction is much stronger than ordinary dipole–dipole interactions.

Types of Hydrogen Bonding

  1. Intermolecular Hydrogen Bonding

Occurs between molecules.

Examples:

  • Water (H₂O)
  • Hydrogen fluoride (HF)
  • Ammonia (NH₃)

This leads to higher boiling and melting points.

  1. Intramolecular Hydrogen Bonding

Occurs within the same molecule.

Examples:

  • Ortho-nitrophenol
  • Salicylic acid

This affects solubility and vapour pressure.

Examples of Hydrogen Bonding in Compounds

Water (H₂O)

Each water molecule forms hydrogen bonds with up to four other molecules.
This is why water has:

  • High boiling point
  • High specific heat
  • High surface tension

Ammonia (NH₃)

Hydrogen bonds form between hydrogen and nitrogen atoms of neighbouring molecules.

Hydrogen fluoride (HF)

Strong hydrogen bonding leads to high boiling point.

Properties Explained by Hydrogen Bonding

Hydrogen bonding explains many unusual properties:

  1. High boiling and melting points

Water has a high boiling point because hydrogen bonds require a lot of energy to break.

  1. Ice is less dense than water

Hydrogen bonds form an open structure in ice, making it float.

  1. High surface tension of water

Water droplets form beads because of strong hydrogen bonding.

  1. Solubility

Water dissolves many polar substances due to hydrogen bonding.

  1. Viscosity and physical states

Hydrogen bonding increases the thickness and stickiness of liquids like glycerol.

Role of Hydrogen Bonding in Biology

Hydrogen bonding plays a vital role in living organisms:

DNA Structure

Hydrogen bonds hold the two strands of DNA together.

  • A–T pairs have 2 hydrogen bonds
  • G–C pairs have 3 hydrogen bonds

This gives DNA its double-helix shape.

Protein Folding

Proteins maintain their shape because of hydrogen bonds in their structure.

Water in the Body

Many properties of water essential for life (heat regulation, nutrient transport) depend on hydrogen bonds.

Hydrogen Bonding in Everyday Life

Hydrogen bonding explains:

  • Why water has a high boiling point
  • Why ice floats
  • Why alcohol evaporates quickly
  • Why hair proteins form curls
  • Why cotton absorbs water
  • Why water has strong surface tension

These examples show its importance in daily life.

Conclusion

Hydrogen bonding is a strong intermolecular attraction between a hydrogen atom attached to an electronegative atom and another electronegative atom with a lone pair. It occurs mainly in molecules containing O–H, N–H, and F–H bonds. Hydrogen bonding is responsible for many unique properties of water, the stability of biological molecules, and important physical behaviours of substances. It is essential for understanding molecular interactions in chemistry and life processes.