What is the structure of a water molecule?

A water molecule consists of two hydrogen atoms and one oxygen atom. The three atoms make an angle; the H-O-H angle is approximately 104.5 degrees. The center of each hydrogen atom is approximately 0.0957 nm from the center of the oxygen atom. The structure of a single water molecule is shown below:

The pictures on this page are provided courtesy of the MathMol project at the NYU/ACF Scientific Visualization Laboratory.
Information about MathMol can be found here.

Because oxygen is more electronegative than hydrogen (in other words, electrons tend to be in the neighborhood of the oxygen), the hydrogen atoms end up with a partial positive charge and the oxygen atom with a partial negative charge. This separation of charge produces a net dipole moment on the molecule; for the isolated water molecule this dipole moment is approximately 1.85 Debye units.

This molecular structure leads to hydrogen bonding, which is a stabilized structure in which a hydrogen atom is in a line between the oxygen atom on its own molecule and the oxygen on another molecule. This picture shows a hydrogen-bonded structure between two water molecules:

These hydrogen bonds, with their extra attractive energy, are the cause of many of the unusual properties of water, including its large heat of vaporization and its expansion upon freezing.

Updated December 3, 2013