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:
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.