In the center of the third row of the periodic table sits silicon,
with four outer electrons like carbon. The rocks of our planet are
derived from silicon dioxide, SiO2,
and are surrounded by an atmosphere composed in part of carbon dioxide
This does not seem remarkable until we recognize that silicon and
carbon, which have the same outer electronic structure, should have
similar chemical properties. Why the gross difference in properties
of their oxides?
The difference in properties arises because a silicon atom is larger
than a carbon atom. Sharing two electron pairs with another atom
in a double bond requires a closer approach of atoms than for a
Silicon, with an inner core of ten electrons, cannot get close
enough. Carbon, with a two-electron inner core, is smaller and can
make C=O bonds. Two such double bonds build a CO2
Rather than making double bonds to two oxygen atoms, it is easier
for silicon to make single bonds to four oxygens, arranged around
the Si atom at the corners of a tetrahedron.
Each of these oxygen atoms can bridge two silicon
atoms, and the result is an endless threedimensional lattice of
silicate tetrahedra as in quartz, shown on the next page.
If silicon were smaller and could make double bonds
to oxygen, there would be no reason not to expect discrete molecules
of O=Si=0. Quartz would be a gas instead of a very hard mineral,
and the history of our planet would be vastly different.
There is another factor: the smaller electronegativity
of Si as compared with 0, 1.8 versus 3.5.