Electronegativity.

A chemical bond is the energy that holds two atoms together in a compound. This energy can be further broken down into ionic bonding and covalent bonding. An ionic bond is a chemical bond characterized by attraction between ions of opposite charge. The formation of an ionic bond involves a complete transfer of electrons between atoms, and can be predicted when one atom has a much higher electronegativity than the other. A covalent bond is …

…sometimes described as "dipoles", meaning that the molecule has two "poles". One end of the molecule has a partial positive charge while the other end has a partial negative charge. The hydrogen bond is really a type of dipole forces. A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine.