Linus Pauling: This lecture is the first of three lectures on one part of chemistry - valence and
the structure of atoms, molecules, and crystals. It was the British scientist Eddington
who said that the study of the physical world, and I would say the biological world
too, is a search for structure and not a search for substance. If we want to understand
the human body, we must know its structure in terms of the cells that make it up.
If we want to understand cells, we must know their structure in terms of molecules.
If we want to understand molecules, we must know their structure in terms of atoms.
And to understand atoms, we must know their structure in terms of electrons and nuclei
- one nucleus down in the center of the atom with a number of electrons around. To
understand nuclei, as the physicists are trying to do now, we must know, must learn
their structure in terms of protons and neutrons and perhaps mesons. And it may well
be that at sometime in the future, the fundamental particles, the electrons, protons,
neutrons, mesons, will be found to have structure also in terms of still smaller particles
that have not yet been discovered.
This subject - a branch of chemistry that correlates the multitude of facts of chemistry
in an effective way and makes it easier for the student to learn and to remember the
subject - this branch of chemistry is illustrated by the model that I hold here; a
model showing the structure of ice. Here we have the hydrogen atoms, the oxygen atoms,
the water molecules arranged together in space in a certain way. It is this arrangement,
this structure, that accounts for the properties of ice. I hope to be able to come
back to discussion of the structure and properties of ice later on.
Now, let us consider some substances and their properties that need to be explained
in terms of structure. You all know the example of diamond and carbon. Both of these
substances are made up of carbon atoms only. Diamond and graphite. Here I have a
diamond. It is the hardest substance known. Here we have a little octahedral crystal,
as they grow in nature. This came from South Africa. This substance will scratch
any substance that we know about. Then there is the other form of carbon - graphite.
This is a chunk of natural graphite which is very soft, so soft that my fingernail
will scratch it. I can, it is used, as you know, in lead pencils for writing. If
I rub this piece of graphite across the paper, it, some of the pieces, some of the
bits of graphite tear off and leave their impression there on the paper. Later on,
we shall discuss the arrangement of the atoms in diamond and graphite and see how
this arrangement explains their properties.
Then, I have here, two crystals. Cleavages, cleavage specimens, from naturally occurring
crystals. Salt, sodium chloride, which, when it is hit or cracked with a hammer,
it breaks along planes at right angles to one another, And Iceland spar, calcite,
CaCO3, calcium carbonate, which breaks along plains that are not at right angles to each
other. This property of cleavage is determined by the structure of the crystal.
Another interesting example of correlation between structure and properties that we
now understand really, essentially, completely, is provided by the silicate minerals.
Here I have a natural crystal of feldspar, autocleaved [?] feldspar. The formula
of this feldspar is KAlSi3O8. It is a potassium aluminum silicate, alumino silicate. These other crystals, beryl,
Be3Al2Si6O18, garnet, Mg3Al2Si3O12, tourmaline - I can’t remember the formula of tourmaline right at the moment - silica,
quartz, SiO2. These substances are hard and strong. They are the princi- quartz and feldspar
are the principle constituents of the heterogeneous material granite. The pink here
is feldspar, the white grains, the white phases are crystals of quartz.
In this case, the atoms are held together by bonds that go in all directions so that
the crystals are hard.