Unit 3: Chemical Bonding

Section 4: More About Bonding

* Hydrogen Bonds
* Forces Between Nonpolar Molecules

Hydrogen Bonds

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.

Forces Between Nonpolar Molecules

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.