A hydrogen bond is a type of chemical bond that weakly attaches a molecule to another molecule. It is based on the attraction between opposite electric charges. The negative charge on an electronegative atom of one molecule is attracted to a positive charge on a hydrogen atom of another molecule. The hydrogen atom carries a positive charge because it is bonded to an second electronegative atom, which shifts electrons away from the hydrogen. This type of bond always involves a hydrogen atom, and two electronegative atoms. The electronegative atoms are often oxygen or nitrogen. Hydrogen bonds are important in polar solvents such as water and alcohol, in biomolecules, and in many other materials.