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The Hydrogen Bond

Nitrogen, oxygen and fluorine are the three most electronegative elements. When they are attached to a hydrogen atom, the electrons are drawn towards the electronegative atom.
The H atom attached to nitrogen, oxygen or fluorine is able to interpose itself between two such electronegative atoms bonding them together. This is called hydrogen bond.
The essential requirements for a H-bond are:
  1. a hydrogen atom attached to a higher electronegative atom;
  2. a lone pair of electrons on the electronegative atom.
The hydrogen bond is written as
A - H ...B or A - H ...A
where A and B represent O, N or F; A - H is one molecule or part of a molecule and B is a part of another molecule and the dotted line represents the hydrogen bond. The three atoms usually lie in a straight line, but the angle AHB (or AHA) can deviate as much as 30o from linearity. Note that O, N and F atoms all possess at least one lone pair of electrons that can interact with the hydrogen atom in hydrogen bonding.

The energy of a hydrogen bond is of the order 40 kJ/mol. Hydrogen bonds have a powerful effect on the structures and properties of many compounds.

The boiling points of a series of similar compounds containing elements in the same periodic group normally increase with increasing molar mass. The hydrogen compounds of the elements in Groups 15, 16 and 17 do not follow this trend. In each of these series, the lightest compound (NH3, H2O, HF) has the highest boiling point, contrary to expectations based on molar hydrogen bonding between molecules in these compounds.

Some unique properties of water such as the density of ice being less than that of water and the contraction of water when heated between Oo C and 4oC can also be explained on the basis of hydrogen bonding.

The inter molecular hydrogen bonding present in H2O, HF can be represented as follows:
H ----O -----H------O-----H : H-----F

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