Page 156 - The Creation Of The Universe

154 THE CREATION OF THE UNIVERSE

around the nucleus. The orbit closest to the nucleus can be occupied by
no more than two electrons. In the next orbit a maximum of eight electrons
is possible. In the third orbit, there can be up to eighteen. The number of
electrons continues to increase with the addition of more orbits. Now an
interesting aspect of this scheme is that atoms seem to "want" to complete
the number of electrons in their orbital shells. Oxygen, for example, has six
electrons in its second (and outermost) orbit, and this makes it "eager" to
enter into combinations with other atoms that will supply the two more
electrons that are needed to increase this number to eight. (Why atoms be-
have this way is a question that is unanswered. But it's a good thing they
do: because if they didn't, life wouldn't be possible.)
Covalent bonds are the result of this tendency of atoms to complete
their orbital shells. Two or more atoms can often make up the shortfall in
their orbits by sharing electrons with one another. A good example is the
water molecule (H O), whose building-blocks (two hydrogen atoms and
2
one oxygen atom) form a covalent bond. In this compound, oxygen com-
pletes the number of electrons in its second orbit to eight by sharing the
two electrons (one each) in the orbital shells of the two hydrogen atoms;
in the same way, the hydrogen atoms each "borrow" one electron from
oxygen to complete their own shells.
Carbon is very good at forming covalent bonds with other atoms (in-
cluding carbon atoms) from which an enormous number of different com-
pounds can be made. One of the simplest of these compounds is methane:
a common gas that is formed from the covalent bonding of four hydrogen
atoms and one carbon atom. With only six electrons, carbon's outer orbital
shell is short of the eight that it needs by four, rather than two as is the
case with oxygen, and for this reason, four hydrogen atoms are needed to
complete it.
We said that carbon was especially versatile in forming bonds with oth-
er atoms and this versatility makes an enormous number of different com-
pounds possible. The class of compounds formed exclusively from carbon
and hydrogen are called "hydrocarbons". This is a huge family of com-
pounds that includes natural gas, liquid petroleum, kerosene, and lubricat-