hydrogen bond
/ HY-druh-jen bond /
Picture tiny magnets so weak that a single one barely holds, but a million of them together can lift something heavy. That is the spirit of the hydrogen bond: individually feeble, collectively decisive.
A hydrogen bond forms when a hydrogen atom already attached to a strong electron-puller (usually oxygen or nitrogen) is also weakly attracted to another nearby electron-puller. Because that hydrogen carries a slight positive charge and the neighbor a slight negative charge, they reach for each other. It is not a true chemical bond that fuses atoms into a molecule — it is a much weaker attraction between molecules, or between distant parts of one large molecule. Each one is easy to break, which is exactly why they are so useful: they let structures hold together yet still rearrange.
Hydrogen bonds are everywhere in biology. They link water molecules (giving water all its special properties), zip the two strands of DNA together while still allowing them to be peeled apart for copying, and pin proteins into their working shapes. Life depends on a bond strong enough to organize and weak enough to undo.
Water droplets bead up on a waxy leaf because the water molecules cling to each other by hydrogen bonds rather than spreading out onto the nonpolar surface.
Surface tension — beading droplets — is hydrogen bonding you can see with the naked eye.
A hydrogen bond is far weaker than the covalent bonds that hold atoms inside a molecule (roughly a twentieth as strong); confusing the two leads to wrong predictions about what breaks first.