A hydrogen atom normally forms a single bond. In some compounds, however, the hydrogen atom may be located between two atoms acting as a bridge between them. Hydrogen atom is now involved in two bonds, one a normal covalent bond and the other a hydrogen bond. A hydrogen bond is always formed between two small, strongly electronegative atoms such as fluorine, oxygen, and nitrogen.
Intermolecular hydrogen bonding: Molecular association
When the hydrogen bond exists between two molecules of the same compound or of different compounds, the hydrogen bond is called intermolecular hydrogen bond. It leads to association of a large number of molecules to form a cluster.For example, hydrogen fluoride, water, ammonia, alcohols and phenols, and carboxylic acids.
Intramolecular hydrogen bonding
Sometimes hydrogen bonding may take place within a molecule. This is known as intramolecular (or internal) hydrogen bonding. It may lead to the linkage of two groups to form a ring. Such an effect is known as chelation. Some examples are complex compounds such as o-nitrophenol, o-hydroxybenzaldehyde, o-chlorophenol, and o-hydroxybenzoic acid.