When H+ (a hydrogen ion) and OH- (a hydroxide ion) combine, they form a covalent bond to create a water molecule (H2O). This bond is known as a hydrogen bond. Hydrogen bonds are formed when a hydrogen atom covalently bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine) interacts with another highly electronegative atom. In the case of water, the hydrogen atom in the OH group forms a hydrogen bond with the oxygen atom in the H2O molecule. Hydrogen bonds are responsible for many of the unique properties of water, such as its high surface tension and high specific heat capacity.