#"Hydrogen bonding"# is an intermolecular bonding phenomenon that occurs when hydrogen is bound to a strongly electronegative element such as oxygen or fluorine or nitrogen. The heteroatom polarizes electron density away from the bond, giving a slightly positive charge to #H#, and a slightly negative charge to the heteratom, e.g. #""^(delta+)H-F^(delta-)#.
In bulk solution, these dipoles line up and constitute a net intermolecular force. The elevated boiling points of water, and hydrogen fluoride are attributed to this intermolecular hydrogen bonding. Ammonia also has an elevated boiling point for this reason.
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