Structure of water and hydrogen bonding

A water molecule is one oxygen atom bonded to two hydrogen atoms, but it is not a straight line. The two O–H bonds meet at the oxygen at an angle of about 104.5°, so the molecule is bent, like a wide V with oxygen at the point.

Inside each O–H bond the two atoms share electrons, but they do not share them evenly. Oxygen pulls the shared electrons much harder than hydrogen does, so the electrons spend more time near the oxygen. That leaves the oxygen end slightly negative (written δ⁻) and each hydrogen end slightly positive (δ⁺). Unequal sharing like this is what makes a bond polar.

Here is the part students miss: unequal sharing alone is not enough. Because the molecule is bent, the two bond dipoles point the same general direction and add up to one net dipole across the whole molecule. If water were linear and symmetric, those two pulls would cancel and the molecule would be nonpolar. It is the combination of unequal sharing and a bent shape that gives water a permanent, lopsided charge — a molecular magnet with a negative end and a positive end.

This is the single most important distinction in the whole topic, and the one that trips up the most students. There are two kinds of attraction in the picture above, and they are not the same thing.

1. The covalent O–H bond — inside one molecule. This is the intramolecular bond that holds a single water molecule together: oxygen and hydrogen sharing electrons. It is strong. It is not broken when water boils or freezes.
2. The hydrogen bond — between separate molecules. This is the intermolecular attraction between the δ⁺ hydrogen of one water molecule and the δ⁻ oxygen of a different water molecule. Any one hydrogen bond is weak — far weaker than a covalent bond — and it breaks and reforms constantly in liquid water.

Two warnings. First, a “hydrogen bond” is not just any bond containing a hydrogen atom — the covalent O–H bond contains hydrogen and is not a hydrogen bond. Second, when water boils, it is the weak hydrogen bonds between molecules that break so molecules can fly apart as vapor; the covalent O–H bonds stay intact, which is why steam is still H₂O. Keep “inside = strong covalent” and “between = weak hydrogen bond” straight and most Unit 1 water traps fall apart.

Quick reference card. Keep straight what happens inside a molecule versus between molecules.

Every “special” property of water comes from the same source: many weak hydrogen bonds acting together. No single hydrogen bond is strong — the power is in the collective network. This is what “emergent” means: the property belongs to the whole, not to any one bond.

Cohesion and surface tension. Because water molecules hydrogen-bond to each other, they stick together (cohesion). At the surface, molecules bond to their neighbors and form a taut film — surface tension — strong enough that a water strider can stand on a pond. It is not one strong bond under each leg; it is the whole surface network.

Adhesion and capillary action. Water also hydrogen-bonds to other polar surfaces (adhesion). Adhesion to the walls of a plant's narrow vessels, plus cohesion pulling the water column along, lets water climb upward against gravity — capillary action.

High specific heat. When you heat water, the energy first goes into loosening the huge web of hydrogen bonds between molecules before it can speed the molecules up. So water's temperature rises slowly and it resists temperature change — it can absorb a lot of heat for a small change in temperature. (The covalent O–H bonds are not what absorbs this heat.)

Why ice floats. As water freezes, each molecule locks into a rigid, open hydrogen-bonded lattice that holds molecules farther apart than they sit in liquid water. More space per molecule means lower density, so solid water floats on liquid water. Water is the exception to the usual rule that solids are denser than their liquids — and it is why a pond freezes from the top down, leaving life alive beneath the ice. The same expansion is why a sealed can bursts in the freezer.

Structure → function: the universal solvent. Because water is polar, its partial charges surround and pull apart the ions and other polar substances they meet — the δ⁻ oxygen tugs at positive ions, the δ⁺ hydrogens at negative ions. That is why water dissolves salts like NaCl and countless polar biological molecules, making it the universal solvent of living cells. Change the structure — make water nonpolar or straighten it out — and every one of these functions disappears.