the hydrogen bond

There is one secondary bond much stronger and more particular than ordinary van der Waals attraction: the hydrogen bond. It appears when a hydrogen atom is already covalently bonded to a very electronegative atom (oxygen, nitrogen, or fluorine) and reaches across to a second such atom nearby. It is the special bond that makes water behave so strangely, holds the two strands of DNA together, and stiffens proteins and polymers.

Here is the mechanism. When hydrogen bonds to, say, oxygen, the greedy oxygen pulls the shared electrons strongly toward itself, leaving the hydrogen's nucleus, a bare proton, poorly shielded and noticeably positive. That exposed positive hydrogen then attracts a lone pair of electrons on a neighbouring oxygen or nitrogen. Because a lone pair points a definite way, the hydrogen bond is directional (unlike plain van der Waals bonding), and it is stronger than ordinary secondary bonds, about 0.1 to 0.5 eV, intermediate between the feeble van der Waals bond and a full covalent bond.

Hydrogen bonds punch above their weight in structure. Each water molecule can hydrogen-bond to four neighbours in a directional, open tetrahedral pattern; that openness is exactly why ice is less dense than liquid water and floats, a rare and life-critical anomaly. The same directional bonds zip DNA's base pairs, fold proteins into their working shapes, and cross-link polymer chains to raise their strength and melting point. Where van der Waals forces merely stack, hydrogen bonds can build specific, directional architectures.