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Some atomic assemblies are
simple. As we have seen, hydrogen atoms stick to each other to form the
(H2) molecule. Hydrogen atoms stick
to oxygen atoms to form the water (H2O)
molecule, to nitrogen to form ammonia (NH3)
and to carbon to form methane (CH4)
- as shown on the left. Oxygen binds to carbon to form carbon
monoxide (CO) and
carbon dioxide (CO2), also shown on the left).
These and many other compounds based on small molecules are immensely
important; for example, water makes up 80% of our body weight and methane
is a widely used fuel (being the main constituent of natural gas).
CO2 has been implicated in global warming - this
small molecule, produced in vast quantities, could change the climate of
the world.
H2, H2O, NH3 and CH4
CO and CO2
The architectural possibilities in small molecules are, however, necessarily
limited. To achieve complex structures, there are a number of simple games
we can play with atoms. The first is to form chains. And the element
which uses this trick most widely is carbon. The simplest carbon chain
compounds are the paraffins - chains of carbon atoms with hydrogens bonded
to the carbon shown on the left.
Paraffin molecules
The chain may be straight or branched as
seen in the figure. Moreover, there are no restrictions on their length,
and the properties of the resulting compounds vary with length. Paraffins
containing 2, 3, or 4 molecules are gases under normal conditions (and
are widely used as fuels). From 5 to 8 carbon atoms, we have light fluids.
(Petrol/gasoline comprises molecules of this size.) As the chain length
increases, the liquid becomes more viscous and for longer chains (greater
than approximately 20 carbon atoms)waxes and tars are formed. These simple
and straightforward chain compounds, whose structures are perhaps slightly
dull, are nevertheless of great importance in modern society. The molecules
are rich in energy; and the energy can be released when they react with
oxygen present in air, which is, of course, the basis of their widespread
use as fuels.
!--Typical polymer molecules-->
Chain molecules are also
widely produced by the modern chemical industry. Indeed the plastics and
fibers which are so widely used today are polymers. Such chains
contain a basic unit - a small group of atoms which is repeated tens, hundreds
and thousands of times. Again the properties of the resulting materials
depend upon the chain length.Typical and important polymers
are illustrated on the below.
Typical polymer structures.
Polymer chain molecules, of course, pack
together in liquids and solids - a topic to which we will return later.
An intriguing form of chemical interaction between polymers is ''cross-linking'
where atoms from one polymer chain form chemical bonds with those in neighboring
chains; a typical example is shown here.
Cross linking in polymers.
Such processes may dramatically
alter the physical and chemical properties of the polymer compound.
By far the most sophisticated,
intriguing and important chain molecules are those present in living matter.
Indeed, living matter has perfected
the art of designing chain molecules for precise and particular chemical
and physical tasks.
Moreover, we again find that living
matter uses rings of atoms. For example, sugar molecules - one of the main
fuels of animals and plants - are built from rings of carbon and oxygen
atoms as illustrated.
The next strategy in achieving
complex structures is to extend into two and three dimensions. Such structures
will be considered in Atomic Scale Architecture. Meanwhile, having gained some
understanding of molecular architecture, we consider the next obvious question,
that is, how molecules interact with each other.
Ring structures.
Sugar structures.
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