Oxygen
There are also electrostatic forces (Van de'r Waals forces) between the hydrogen atoms and adjacent molecules' oxygen atoms. These pseudo-bonds bring the atoms around 15% closer to each other than most other simple liquids. This is because Water is a polar molecule (Net asymmetrical distribution of electrons) due to its bent shape, giving it an overall net field direction, mainly due to oxygens 2 non bonding pairs of electrons, pushing the bonding H's further together than the linear arrangement with lower enthalpy (see CO2). This property is exploited by microwaves to oscillate polar molecules, especially water. And its responsible for the extra energy needed to disassociate H20.