What is hydrogen bond? How is it formed?

A hydrogen bond is defined as the weak electronic force of attraction which exists between the covalently bonded hydrogen of one molecule and highly electronegative atom of another molecule. Hydrogen bond has a strength of the order of 10 – 100 kJ mol^–1as compared to another covalent bond which has a strength of the order of 200 – 400 kJ mol^–1. A hydrogen bond is denoted by dotted lines (.....). The hydrogen bonding in hydrogen fluoride is represented as:

Cause for the formation of hydrogen bond: Whenever a hydrogen atom is attached to a highly electronegative atom (for example, N, O, F), the shared pair of electrons between the two atoms is attracted towards the more electronegative atom. Consequently, the highly electronegative atom acquires a partial negative charge (δ^–) while the hydrogen atom acquires a partial positive charge (δ⁺). The partial negatively charged atom of one molecule tends to attract partial positive hydrogen atom of the other molecule. This weak electrostatic attraction constitutes hydrogen bond. For example.