There is a special type of bond that involves a very special atom. When the slightly positive hydrogen atom in a polar molecule react with an very electronegative charge atom, an hydrogen bond forms.
This type of bond is present in water. The slightly positive end of a H2O molecule, hydrogen, is attracted to the slightly negative end of a H2O molecule, oxygen, to form an intermolecular attraction called a hydrogen bond. A hydrogen bond is slightly stronger that dipole-dipole forces due to the fact hydrogen does not have any unshared electrons to interfere in the bonding.
In water, hydrogen bonds accounts for the fact that water is a liquid between 0 degree Celsius and 100 degrees Celsius. A hydrogen bond one tenth the strength as an ionic or covalent bond. Hydrogen bonds are also present in the foundation for life, DNA. The nitrogen bases (adenine, thymine, cytosine, and guanine) are paired up due to hydrogen bonds. The presence of many hydrogen bonds provides a force strong enough to keep the double helix strand together, but yet allows a passageway for the splicing and replication of DNA.
What keeps together nonpolar molecules? Even though nonpolar molecules have dipoles that cancel each other out, there are points where the electrons may not be evenly distributed (i.e. in different locations). At this instant the atom would have a momentary dipole. For example, N2, a nonpolar molecule, forms a liquid at very low temperatures. The N2 molecule is attracted for a very short time to another N2 molecule result in a very weak, but present intermolecular attraction. This temporary attraction are called London forces. London forces account for the fact that nonpolar molecules can form liquids and solids at low temperatures. However, these forces are only one tenth of the strength of dipole-dipole force. These forces can increase when more electrons and larger molecules are present. For example, methane can form a liquid at a higher temperature that N2, but compared to H20, its boiling point is very low. Due to their weak London forces, N2 and CH4 tend to be gases at room temperature, while H20 is a liquid at the same temperature. Dipole-dipole forces and London forces can be considered together and are know as van der Waals forces (name for a scientist who proposed and study their existence). In general, polar molecules tend to have stronger intermolecular bonds than nonpolar molecules. This can lead to a huge difference in the boiling points of polar and nonpolar molecules of similar atomic masses. The polar molecule would have a higher boiling point.
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Unit 3 - Section 3