V.+Hydrogen+bonding

The **hydrogen bond** is 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. **
 * 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, 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 weak force is called a **hydrogen bond**. The hydrogen atoms of one water molecule stick to the oxygen atoms of nearby water molecules, freezing them together into solid ice.
 * In a water molecule the electron shell round a hydrogen atom is rather thin, and the positive charge on its nucleus shows through to the outside world, giving the hydrogen atom a small positive charge. On the other hand the electron shell round an oxygen atom is quite thick, and so oxygen carries an extra bit of negative charge. These opposite charges attract, although quite weakly.

Example) You can float a paper clip on the surface of a glass of water. Before you try this you should know that it helps if the paper clip is a little greasy so the water doesn't stick to it (rub it on your nose or forehead.) Place the paper clip on a fork and lower it slowly into the water. The paper clip is supported by the surface-tension skin of the water.