Agriculture Reference
In-Depth Information
SOIL COMPONENTS INTERACTING
2.2.
BONDING CONSIDERATIONS
As noted in Chapter 1, sand, silt, clay, and organic matter do not act indepen-
dently of each other in soil. Thus one or several types of chemical bonds or
interactions—ionic, polar covalent, covalent, hydrogen, polar-polar interac-
tions, and Van der Waals attractions—will be important in holding soil com-
ponents together. The whole area of chemical bonding is extremely complex,
and thus, in addition to specific bonding considerations there are also more
general ways of investigating the interaction between components in soil.
These involve, for instance, graphing the adsorption of organic compounds to
soil constituents at increasing levels of organic compound. The shape of the
graph is used as an indication of the type(s) of interaction(s) between con-
stituents at various concentrations.
2.2.1.
Orbital Overlap
Interaction between silicon or aluminum surfaces or edges and surrounding
components will entail overlap of surface orbitals and available orbitals in the
approaching species forming covalent or polar covalent bonds. The strength
of the interaction will depend on the strength of overlap of the available
orbitals. Bonding energies of orbitals are well known, and so the strength of
the bonds can be either directly known or estimated. Orbital overlap is not
the only factor affecting the interaction or strength of bonding. In addition to
energy, reaction path, steric, and rate factors will play a role in any attraction.
Orbitals can also be considered from a molecular orbital standpoint. Each
reacting species must have molecular orbitals available, and be of the correct
symmetry such as to allow for bonding. These will be called the “frontier
orbitals,” composed of the highest occupied (HOMO) and the lowest unoc-
cupied (LUMO) molecular orbitals. In addition to their involvement in
bonding between species, these orbitals are of considerable interest in that
they are largely responsible for many of the chemical and spectroscopic char-
acteristics of molecules and species and are thus important in analytical pro-
cedures and spectroscopic methods of analysis [4-6].
2.2.2.
Ionic Bonding
Ionic bonding is generally regarded as the predominant type of bonding
between the ions that make up salts or the compounds formed between metals
and nonmetals. The basic concept is always illustrated as a compound such as
sodium chloride and explained by saying that sodium donates its outer most
electron to chlorine such that both have a noble gas electron configuration.
