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Definition of hydrogen bond?
A hydrogen bond is the attractive force between the hydrogen attached to a more electronegative atom of one molecule and another more electronegative atom of a different molecule. Hydrogen bonds form between polar molecules, in which the more electronegative element gains a partially positive negative charge (##delta^-##) and the less electronegative hydrogen atom gains a partially positive charge (##delta^+##). This happens because the shared electrons tend to be held more closely to the more electronegative atom. An example of this is water ##"H"_2"O"##.
The oxygen atom is more electronegative than the hydrogen atoms, so the oxygen atom holds its own bonded electrons and the hydrogen electrons more closely, developing a slight negative charge, while the hydrogen atoms develop a slight positive charge. Because of this, water molecules form hydrogen bonds with one another.
Notice that a hydrogen atom attached to the more electronegative oxygen atom of one water molecule forms an attractive force, a hydrogen bond, between itself and the more electronegative oxygen atom of another water molecule.
Hydrogen bonding occurs between other molecules in nature. In fact, the double helix of DNA is held together by hydrogen bonds between the nitrogen bases of each strand.