A single molecule of water consists of three atoms. H2O is built from a single atom of oxygen that is single-bonded with two atoms of hydrogen.
Oxygen is generally eager to bind with hydrogen. These atoms bond strongly with each other, and the resulting water molecule is unusually stable and resistant to decomposition. Within each water molecule, oxygen is bound to hydrogen by a covalent bond consisting of two shared electrons per hydrogen atom. Oxygen's other, unshared, electrons tend to push against the hydrogen atoms and force them toward one side of the oxygen, resulting in an asymmetrical electromagnetic polarity that permits weaker hydrogen bonds between molecules.