Hydrogen+Bonding.

__Hydrogen Bonding.__



Hydrogen bonding occurs between molecules which contain hydrogen directly bonded to fluorine, nitrogen or oxygen. Hydrogen bonding is the type of dipole-dipole attraction, which it has big amount of electronegativity difference between and above mentioned molecule.

Hence, these bonds are the strongest form of intermolecular force, and thus they have a much higher boiling point than expected from their Mr. For example if it water did not have a hydrogen bond, it would be a gas at room temperature. Specific examples can support this idea. CH4-O-CH3 (Methoxymethane), which does not form hydrogen bond, has Mr of 46, and its boiling point is -23 Celcius. In contrast, CH3CH2-O-H (Ethanol), which does form hydrogen bond, has Mr of 46, and its boiling point is 79 Celcius (way higher than previous one).

Water is also a special case. Because of the hydrogen atoms and two lone pairs of oxygen atoms in water, it is possible for each H2O to form four hydrogen bonds with neighbouring molecules in its solid form. This results in a tetrahedral arrangement meaning that the molecules are a fixed distance apart. So there it is an open structure, and therefore solid H2O is more dense than liquid. This is why ice floats on top of water.

It is also possible to have hydrogen bonds within large molecules, for example protein. The hydrogen bonds of large molecules is an important characteristic for determining their properties.