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Nitrogen, oxygen and fluorine are highly electronegative elements. When they form a covalent bond with hydrogen atom, due to difference in electronegativity, dipoles are created where hydrogen atom acquires a partial positive charge and the electronegative atom acquires a partial negative charge. This partially positively charged hydrogen atom forms a weak bond with another highly electronegative atom nearby as depicted below :
This weak bond, usually represented by dotted lines, is known as hydrogen bond.
Chlorine having the same electronegativity as that of nitrogen atom does not form hydrogen bond due to its large size.
There are two types of hydrogen bonds.
Intermolecular hydrogen bond is formed between two different molecules of the same or different compounds.
Examples of intermolecular hydrogen bonding (different molecules of the same compound) : HF, R-OH (R is alkyl group), H2O etc.
Intermolecular hydrogen bond in hydrogen fluoride (HF) ↓
Intermolecular hydrogen bond in alcohol molecule↓
Examples of intermolecular hydrogen bonding in different compounds : 'Water and alcohol' , 'Ammonia and water' etc.
Intermolecular hydrogen bond between ethanol and water
Intramolecular hydrogen bond is formed when hydrogen atom is in between the two highly electronegative atoms present within the same molecule. Example : In salicylic acid, hydrogen is attached to one oxygen with a covalent bond and to other with intramolecular hydrogen bond.
Compounds that are capable of forming hydrogen bond tend to show abnormality in their physical properties. Some effects of hydrogen bond are given below :
Intermolecular hydrogen bonding in water
Many carboxylic acids exist as dimer due to intermolcular hydrogen bonding.
Intermolecular hydrogen bond in carboxylic acid↓