hydrogen bond
/ HY-druh-jen bond /
Picture a tiny hydrogen atom that has already lent most of its single electron to a hungry neighbor like oxygen. Stripped of its electron, the hydrogen is left with a bare positive nucleus poking out, and that exposed positive spot reaches over to nuzzle a nearby atom that has spare negative charge. A hydrogen bond is this modest link: a shared hydrogen bridging two atoms that pull electrons strongly, usually oxygen, nitrogen, or fluorine.
It works because hydrogen is special — it has only one electron, so when that electron is tugged away its nucleus is left almost naked and unusually exposed. The little positive hydrogen then sits between its own atom and a lone pair of electrons on a neighbor, attracted to both. The result is stronger than a fleeting van der Waals tug but much weaker than a true shared-electron bond, and it lines up along a definite direction, so it can act like a flexible but selective fastener.
Hydrogen bonds matter enormously for their size: they hold water molecules together, give water its high boiling point and the strange way ice floats, zip the two strands of DNA into a double helix, and fold proteins into working shapes. The common confusion is to lump them in with ordinary chemical bonds; they are far weaker, easily made and broken at everyday temperatures, which is exactly what makes them so useful for life's constantly rearranging molecules.
Liquid water is full of hydrogen bonds: the hydrogen on one molecule reaches over to the oxygen on its neighbor. When water freezes, these bonds lock the molecules into an open, roomy crystal that takes up more space than liquid water — which is why ice is less dense and floats, a quirk almost no other substance shares.
Hydrogen bonds hold water molecules in an open lattice, so ice floats.
A hydrogen bond is not the bond that ties hydrogen to its own oxygen inside a water molecule — that inner one is a strong covalent bond. The hydrogen bond is the weaker link reaching out to a separate, neighboring molecule.