Introduction to Soil Chemistry

36 soil basics ii
s p d sp 3
s–s s–p p–p px –px d–d
s–sp 3
Figure 2.4. Common representations of the s, p, d, and atomic orbitals; sp 3 -hybridized orbitals and
some representations of how they overlap to form bonds between atoms are also shown.
bonding angle between oxygen and hydrogen, the partially negative oxygen
will also be exposed to the medium; and (3) surfaces with broken edges, which
can present a number of different orbitals depending on where the break
occurs.
It can be imagined that the bonds at edges can be broken at any given location,
that is, with an oxygen, hydroxy, silicon, or aluminum exposed. In this case
it could further be imagined that s-, p-, and sp 3 -hybridized orbitals would be
on the surface. This will lead to complex bonding and reactivity, resulting in
bonds of varying strengths and interactions and of varying types. The issue is
then how these surfaces will interact with components commonly found in the
soil solution.
Molecular orbital depictions of the orbitals described above are given in
Figure 2.4. To the newcomer, these types of diagrams can be confusing. The
orbitals and their shapes are calculated. The calculations result in the orbitals
having charge signs, plus (+) and minus (-), and will often be represented in
this fashion. It is common to think of negative and positive signs as representing
negative and positive charges with negative and positives attracting
each other and two negatives or two positives repelling each other. However,
the signs (+, -) in this case do not represent charges. In molecular orbital diagrams
the overlap of two atomic orbitals having the same sign denotes a positive
interaction leading to bonding, that is, holding the two atoms together.
This can also be indicated by two orbitals having the same shading as shown
in Figure 2.4.
When the two orbitals have different signs, they do not overlap (nor do they
cancel each other out) but result in the formation of antibonding orbitals.
In this case the electrons are not shared between two atoms and do not
hold the atoms together. For each bonding molecular orbital there is an antibonding
orbital. Antibonding orbitals have higher energy than do bonding
orbitals.
The overlap of p and d orbitals seen in Figure 2.4 can be of two types. They
overlap can be end-on-end as depicted in the p–p and d–d representation, or