BRICKS OF THE UNIVERSE 169
The electrons forming the link appear to have orbits round the
nuclei of both atoms, possibly in a figure-of-eight form. The rapid
intertwined movement of the electrons makes this a very strong link
between the atoms.
USED FOR PUTTING OUT FIRES
These are very simple examples of the linking up of atoms to form
molecules. To take a slightly more complicated case: the carbon atom
has four electrons on its outermost shell, so that it can link up with
four atoms of chlorine. One electron of the carbon atom forms a link
with an electron of one of the chlorine atoms; the other three chlorine
atoms are linked to the carbon atom in the same way. Thus we get a
molecule of carbon tetrachloride. Each atom has the inert gas structure;
each of the chlorine atoms gains one electron to make up its eight, and
the carbon atom gains four to make up its eight, one from each chlorine
atom. Thus the compound is sated; that is one of the reasons why it is
used for putting out fires.
Oxygen has six electrons on its outermost shell. An atom of oxygen.
in suitable conditions, can unite with an atom of carbon to form a
molecule of carbon monoxide. Each atom has the inert gas structure;
each atom contributes two electrons to form a double link. The oxygen
atom is sated (6 + 2 = 8) but the carbon still has two free electrons. It
can readily link up with another oxygen atom, and so form a molecule
of carbon dioxide.
In some of the more elaborate organic compounds several thousands
of atoms are combined in a single molecule.
GIANT MOLECULES
THE biggest molecules are so big that we can see and handle them.
They are called giant molecules. The diamond is one of these giant
molecules; any diamond is a single molecule. Instead of the few
thousand atoms that make up an elaborate organic molecule, we have
in a large diamond something like a quadrillion atoms.
WHY A LUMP OF COAL IS NOT A DIAMOND .
A diamond is a. crystal of almost pure carbon. It is the same element
that is found in an impure form in coal and charcoal. The extreme
difference between charcoal and diamond is due to the arrangement and
linkages of the atoms.
Every atom of carbon has four outer electrons, so that it is capable
of joining up with four others. The carbon atom is often described as
tetrahedral, because the links are directed toward the corners of a
tetrahedron, or triangular pyramid. We have to think of each carbon
atom at the centre of a tetrahedron, and linked to four other atoms at
the corners of the tetrahedron. Each of these four atoms is in jturn
linked to four others arranged in the same way. And so on.
B.CXF.—F*