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The models shown so
far of atoms and molecules might lead us to think that
the microscopic world comprises frozen, static
structures. Reality is very different. Atoms and
molecules are in constant motion and even at low
temperatures, matter is very dynamic at microscopic
levels. Molecules constantly rotate and vibrate; they
translate (move bodily) in gases and liquids. They may
also move, but very much more slowly, in solids.
This image shows snapshots
from the motion in a peptide hormone, oxytocin, played in
sequence, as a movie.
Let us look at these
common forms of molecular motion, where we will find once
more that our understanding has been enormously assisted
by the computer. Translation is the simplest. In
gases, which contain low densities of molecules,
molecular motions are remarkably fast. For example, a
hydrogen molecule in the gas under ambient conditions is
traveling on average at 6,940 km/h. Of course it is
constantly bumping into other hydrogen molecules and the walls of
the vessel (and it is the latter collisions that give
rise to the pressure exerted by the gas on the walls of
the vessel). As the temperature rises, the molecules gain
more energy so at 1000 degrees Centigrade on average our
hydrogen molecule will be moving at 14,343 km/h. The
pressure due to the collisions of these more energetic
molecules will be correspondingly higher.
The higher densities
of liquids mean that molecular motions are far slower.
But molecules are constantly on the move in liquids; they
push and jostle their way through their neighbors.
Below, for example, the trajectory of a water molecule
in liquid water over a period of one ten thousand millionth
of a second (10-10 sec) is illustrated. We see once more how
on this time scale, a lot can happen at the molecular
level.
The diffusion of an individual water molecule in liquid water
Molecules in solids
are usually firmly anchored to particular sites.
Diffusion in solids is therefore slow and commonly
requires imperfections or defects in the solid to allow
it to proceed. If, for example, an atom or molecule is
missing from its regular site, a neighboring molecule
can jump into this site allowing diffusion to occur. Such
processes are of central importance to key solid state
reactions including
corrosion. There is, however, an important and exciting
class of solids in which rapid atomic migration takes
place within the solid phase. Indeed, in the important
class of 'fast ion conductors', ions move at speeds in
solids which are typical of those seen in liquids. The
figure shows a computer simulation of the way in which
the lithium ions diffuse in a conducting polymer. The
exposure time is again 10-10 seconds during which one of the ions
(colored pink in the image) moves between many different
sites.
Molecules in gases and
liquids are also constantly rotating as they move. Oxygen
(O2
and nitrogen (N
2) molecules will rotate around
1010 times per
second at ambient conditions. The greater
interference between the molecules will of course slow
down their rotation as well as their translation in the
liquids. More complex molecules show internal rotations
in which only a part (or chemical group) of the molecule rotates.
Such rotations are
commonly 'hindered', that is the rotation of the group is
opposed, by interactions with other parts of the
molecule.
Thirdly, molecules
vibrate just as balls held together by springs vibrate.
Thus the oxygen atom in the oxygen molecule vibrates
(that is the bond shortens and lengthens)
1014 times per
second. The larger the number of atoms, the more complex
the nature of the vibrational motion. The frequency of
a vibration depends on the forces acting on the atom, and
is a constant that does not change with temperature.
However, as the atoms acquire more energy at higher
temperatures, their amplitudes increase, that is,
the extent to which the bonds compress or expand,
increases.
Atoms and molecules
vibrate in solids. Indeed, since translation and rotation
are commonly not possible in the constrained environments
in solids, this is the only type of motion that is
possible. Again the amplitude of the vibrations increase
with temperature. We show a computer simulation of the
vibrational motion in ice close to its melting pint.
The study of atomic
motions - of the dynamics of matter at the atomic
level - is one of the most widely investigated
fields of physical chemistry; and it is one where, as we
have already seen, the computer is playing an
increasingly valuable role. In the next section, we
explore how, in this and in other fields of chemistry and
physics, the computer is a tool of growing power and
importance.
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