Hydrogen bonds are a type of permanent dipole-dipole force that are very strong
They exist between molecules that have a N-H, O-H or F-H bond
Nitrogen, Oxygen and Fluorine are the most electronegative elements and the covalent bonds between them and hydrogen are highly polar
Highly polar bonds result in strong attractive forces between molecules (the slightly negative N, O or F forms hydrogen bonds with slightly positive hydrogen atoms from other molecules)
Substances that can form hydrogen bonds have higher melting points than those unable to
Hydrogen bonding is an intermolecular force that is a particular type of permanent dipole-dipole force.
Nitrogen, oxygen and fluorine are highly electronegative elements. Hydrogen has a very weak electronegativity. Therefore, when hydrogen covalently bonds with either nitrogen, oxygen or fluorine, the bond formed is highly polar.
This high polarity gives rise to permanent dipole-dipole forces between the slightly negative nitrogen, oxygen or fluorine atoms and the slightly positive hydrogen atom. These forces are, relatively speaking, very strong and are called hydrogen bonds.
Hydrogen bonds only occur between molecules that contain nitrogen-hydrogen, oxygen-hydrogen or fluorine-hydrogen bonds.