An hydrogen bond is a dipole-dipole attraction that occurs between a hydrogen atom that has a partial positive charge and an electronegative atom that has a partial negative charge. The maximum number of atoms that can be bonded in an hydrogen bond is three, which occurs in the case of the linear hydrogen bond. This type of hydrogen bond is formed when a hydrogen atom is bonded to two electronegative atoms, such as in the case of water (H2O). The two electronegative atoms compete for the electron density of the hydrogen atom, which creates a partial positive charge on the hydrogen atom and a partial negative charge on the electronegative atoms. This results in a dipole-dipole attraction between the hydrogen atom and the electronegative atoms, which is the hydrogen bond.