decreasing order of strength of hydrogen bonding isp)-O-H-------O-q)-O-H-------N-r)-N-H-------O-s)-N-H-------N-ANS:q>p>s>ras electronegativity increases strength of hydrogen bonding increases. then how the above order is correct?

Dear Anusha,

Hydrogen is a special element. Because it is really just a proton, it turns out that it can form a special type intermolecular interaction called the hydrogen bond. If the hydrogen in a moleucle is bonded to a highly electronegative atom in the second row only (N, O, or F), a hydrogen bond will be formed.

In essence the three elements listed above will grab the electrons for itself, and leave the hydrogen atom with virtually no electron density (since it had only the one). Now, if another molecule comes along with a lone pair, the hydrogen will try to position itself near that lone pair in order to get some electron density back. This ends up forming a partial bond, which we describe as the hydrogen bond. The strength of this interaction, while not quite as strong as a covalent bond, is the strongest of all the intermolecular forces (except for the ionic bond).

A diagram of the hydrogen bond is here:

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