Structure of Water and Hydrogen Bonding

Oxygen has two lone pairs of electrons and two bonds to hydrogen, and all four of these push away from each other, forcing a bent shape instead of a straight line. Oxygen is also more electronegative than hydrogen, so it pulls the shared electrons in each covalent bond closer to itself. That leaves oxygen with a partial negative charge and each hydrogen with a partial positive charge. Because the molecule is bent, these charges do not cancel out, so the whole molecule is polar, with a negative end and a positive end.

That polarity is what lets water molecules hydrogen bond to each other. The partially positive hydrogen on one water molecule gets attracted to the partially negative oxygen on a neighboring molecule. Since oxygen has two lone pairs and two hydrogens, each water molecule can form up to four hydrogen bonds at once, two as the hydrogen donor and two as the oxygen acceptor, linking into a shifting three-dimensional network in liquid water.

This network sets up everything else in the unit: the four carbon-based polymer families all get built and broken using reactions that happen in water, and many of their properties depend on how they interact with this hydrogen-bonded solvent. Water's bonding pattern also explains cohesion, adhesion, high specific heat, and the density behavior of ice, all consequences of the same bent, polar shape.