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We have seen that when two
or more atoms come together they will form chemical bonds because the electron
charge clouds interact and density in the outermost portions is redistributed.
What happens, however, when the molecules interact? Of course, as we saw
earlier, they may react; the bonding patterns rearrange and the atoms find
new partners. New molecules are formed. But molecules will interact even
when they are not forming new chemical bonds. And these 'non-bonding
interactions',
although perhaps less exciting than chemical bonding, are equally important,
and control many of the most important phenomena in nature.
Let us look at the origins
and types of these interactions. The first and simplest are non-bonded
repulsions. Obviously if two molecules get too close together, they
will repel each other: both the nuclei and the electrons will interact
and they will push each other apart. In fact it is these repulsions which
really define the size of the molecule, because they define the space around
a molecule from which other atoms and molecules are excluded.
Non-bonded attractions between
molecules are of two main kinds. The first arise from the fact that
atoms in molecules commonly have significant charges. So positively
charged atoms on one molecule will be attracted to negatively charged atoms
on a second molecule. The classic (and extreme) example of the effect arises
when we have bonding between hydrogen and (highly electronegative) atoms
such as oxygen, nitrogen and fluorine. These latter atoms pull charge towards
them giving a very polarised bond with a positively charged H atom, which
will have to stick to the negatively charged atoms of different molecules.
This strong attractive intermolecular interaction is known as the hydrogen
bond. It is one of the most important forces in nature. Indeed, the
world as we know it and life itself would not be possible without the H
bond. It is the main force acting between water molecules and, indeed,
is responsible for water being a liquid rather than a gas; so without hydrogen
bonding we would have no oceans. It is also a force which plays a crucial
role in the chemistry of proteins and of DNA as we will learn later; so
without hydrogen bonding there would be no life.
The other form of attractive
interaction is more subtle, and arises from a special kind of phasing or
correlation between the motion of electrons. If we allow ourselves
for a moment to think of atoms and molecules using the ideas of classical
physics, we can imagine the electrons orbiting the nuclei. As two atoms
or molecules approach each other we might expect the phasing of these motions
to tend to get in step. Such phased or correlated motions will lead to
attractions as they will allow the electrons on one atom to 'see' the nuclei
on another to a greater extent. These forces, known as 'dispersive' interactions,
act between all molecules. And again, despite their rather undramatic
nature, they are of crucial importance, being the main interaction holding
many solids together.
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