Geology Reference
In-Depth Information
The ionic model of bonding
Few scientists are expected to deal with as wide a
range of materials and properties as the geologist.
Consider the contrast between red-hot silicate lava
and grey Atlantic seawater, between the engineering
properties of crystalline granite and those of soft clay
or mud, between the electrical and optical properties
of quartz and those of gold. The immense physical
diversity of geological materials is derived largely
from the differences in the chemical bonding that holds
them together.
One can distinguish several different mechanisms
by which atoms bond together, although the real
interaction between two atoms is generally a mixture
of more than one bonding type. The extent to which
each mechanism contributes to a real bond depends
on the difference in electronegativity between the
atoms concerned. We begin by examining the type of
bond that predominates when the electronegativity
contrast is large.
The salt sodium chloride, familiar as common table
salt and as the mineral halite, consists of two elements
of notably different electronegativity: 3.2 (Cl) and 0.9
(Na) (difference = 2.3). The low ionization energy of the
sodium atom (ChapterĀ 6) indicates a readiness to lose
an electron, forming a Na + cation. The chlorine atom,
on the other hand, readily accepts an extra electron,
forming a chloride anion Cl - . When a sodium atom
encounters a chlorine atom, one electron may be drawn
from the exposed Na 3 s orbital into the vacancy in the
Cl 3p orbital. The ions resulting from this transfer, hav-
ing opposite charges, experience a mutual electrostatic
attraction that we call ionic bonding .
Electrostatic forces operate in all directions, and an
ion in an ionic compound like NaCl draws its stability
from the attraction of all oppositely charged ions
nearby. Ionic bonding does not lead to the formation of
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