The hydrogen bond can be understood by taking a close look at the water molecule itself. The water molecule is composed of two hydrogen atoms and one oxygenone oxygen atom. The oxygen atom has six valence electrons in its outer shell. These six electrons are distributed into four sp³ orbitals.

Two lone pairs of electrons will occupy the first two sp³ orbitals, leaving an unpaired bonding electron in each of the other two sp³ orbitals. The one and only electron from the hydrogen atom and an unpaired electron from an sp³ orbital occupy a ?-bonding molecular orbital.

The bonding electrons in each ?-covalent bond are localized to the region directly between oxygen and hydrogen nuclei, as indicated by a line, O–H, connecting the two nuclei. The nucleus of the oxygen atom exerts a greater coulombic attraction on the bonding electrons than does the hydrogen nucleus, leaving the single proton that makes up the nucleus of the hydrogen atom partially unshielded. The oxygen atom swells and carries a partial charge of -0.8, while each hydrogen atom carries +0.4 charge. Toggle map of charges on and off.