Hydrogen Bonding I
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The hydrogen bond is really a special case of dipole forces. A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine, which has a partial negative charge. The hydrogen then has the partial positive charge.
To recognize the possibility of hydrogen bonding, examine the Lewis structure of the molecule. The electronegative atom must have one or more unshared electron pairs as in the case of oxygen and nitrogen, and has a negative partial charge. The hydrogen, which has a partial positive charge tries to find another atom of oxygen or nitrogen with excess electrons to share and is attracted to the partial negative charge. This forms the basis for the hydrogen bond.
In other words - The hydrogen on one molecule attached to O or N that is attracted to an O or N of a different molecule.
In the graphic on the left, the hydrogen is partially positive and attracted to the partially negative charge on the oxygen. Because oxygen has two lone pairs, two different hydrogen bonds can be made to each oxygen. This is a very specific bond as indicated. Some combinations which are not hydrogen bonds include: hydrogen to another hydrogen or hydrogen to a carbon. Link to animation of Hydrogen Bonding in Water - Northland Community and Technical College
Hydrogen bonding is usually stronger than normal dipole forces between molecules. Of course hydrogen bonding is not nearly as strong as normal covalent bonds within a molecule - it is only about 1/10 as strong. This is still strong enough to have many important ramifications on the properties of water.
Comparison of Bond Lengths
The graphic on the left shows a cluster of water molecules in the liquid state. Water is a polar molecule, with the oxygen (red) being the negative area and the hydrogen (white) being the more positive area. Opposite charges attract. The bond lengths give some indication of the bond strength. A normal covalent bond is 0.96 Angstroms, while the hydrogen bond length is is 1.97 A.
Electrostatic Potential as an Indication of Polarity
The molecular electrostatic potential is the potential energy of a proton at a particular location near a molecule.
- Negative electrostatic potential corresponds to a attraction of the proton by the concentrated electron density in the molecules (from lone pairs, pi-bonds, etc.) (colored in shades of red).
- Positive electrostatic potential corresponds to repulsion of the proton by the atomic nuclei in regions where low electron density exists and the nuclear charge is incompletely shielded (colored in shades of blue).
The polarity of the water molecule with the attraction of the positive and negative partial charges is the basis for the hydrogen bonding.