Q.Define hydrogen bond. Is it weaker or stronger than the van der Waals forces?

Concept: Hydrogen bonding as a special dipole–dipole interaction; relative strength of intermolecular forces.

A hydrogen bond forms when a hydrogen atom covalently bonded to a highly electronegative atom (F, O, or N) experiences electrostatic attraction to a lone pair on another electronegative atom. The large electronegativity difference creates a significant partial positive charge ($\delta^+$) on hydrogen, allowing it to interact strongly with the electron-rich region of a neighboring molecule.

This interaction is substantially stronger than ordinary van der Waals forces (dipole–dipole, London dispersion) because:

1. The small size of hydrogen allows close approach to the acceptor atom, maximizing electrostatic attraction. …

What is a hydrogen bond?

A hydrogen bond isn't a true chemical bond in the covalent or ionic sense. Instead, it's a special type of intermolecular force that arises under very specific conditions.

The key players are:

• A hydrogen atom covalently bonded to a highly electronegative atom (nitrogen, oxygen, or fluorine)
• Another electronegative atom nearby that has at least one lone pair of electrons

When hydrogen bonds to N, O, or F, the bond is so polar that hydrogen becomes partially positive ($\delta^+$) to an unusual degree—almost like a bare proton with a tiny electron cloud. This exposed positive charge is then attracted to the lone pair on another electronegative atom, creating a bridge between molecules.

We represent it as: $\ce{X-H \cdots Y}$, where $\ce{X}$ and $\ce{Y}$ are N, O, or F, and the dotted line shows the hydrogen bond.

Classic examples:

• Water ($\ce{H2O}$): Each molecule can form up to four hydrogen bonds (two through its hydrogens, two through its lone pairs)
• Ammonia ($\ce{NH3}$): Forms hydrogen bonds through its N–H bonds
• DNA base pairs: Adenine-thymine (2 H-bonds) and guanine-cytosine (3 H-bonds)
• Proteins: Hydrogen bonds stabilize secondary structures like α-helices and β-sheets

Strength comparison with van der Waals forces

Van der Waals forces is an umbrella term covering three types of weak intermolecular attractions:

Hydrogen bonds, by contrast, have energies of 10–40 kJ/mol—roughly an order of magnitude stronger than typical van der Waals forces, though still much weaker than covalent bonds (150–400 kJ/mol).

Why the difference? Hydrogen bonding involves: …