Geology Reference
In-Depth Information
Anion polarizability: non-ideal ionic bonds
Linus Pauling was the first to point out that a contin-
uous progression in bond type must exist between
purely covalent and predominantly ionic bonds. The
character of an intermediate type of bond can be
quantified in terms of the
proportion of ionic character
pre-
sent - expressed as a percentage - which Pauling corre-
lated with the difference in electronegativity between
the participating atoms, as shown in Figure 7.8a.
Although not an exact relationship (and therefore
shown as a band in the figure), the correlation offers a
valuable insight into the properties of minerals.
We have postulated an ideal ionic bond in which an
electron is completely transferred from a donor atom
(which becomes a cation) to a recipient atom (which
becomes an anion). Ideally the transferred electron
should be associated solely with the anion nucleus,
and its spatial distribution should be symmetrical
about it. In reality, the net positive charge on each
neighbouring cation acts as a competing focus of
attraction and to a small extent the electron density of
the anion is concentrated in the region between the
nuclei. In other words, a degree of electron sharing
(partial covalency) occurs in any real ionic bond. The
ideal ionic bond does not actually exist in nature.
The degree of polarization of the anion is obviously
greater if the cation is highly charged (Mg
2+
as compared
with Na
+
, for example). The polarizing field of the cat-
ion is also more intense if the cation is small, because it
can then be brought closer to the anion. The ratio of the
cation's charge to its radius, which is called the
ionic
potential
of the cation, is therefore a measure of its
power to polarize an anion and thereby recover a frac-
tion of the excess electron density. Large singly charged
cations like K
+
possess little polarizing power and form
the 'purest' ionic bonds. Small, multiply charged cations
like Al
3+
and Si
4+
are highly polarizing, and their com-
pounds can be considered partially covalent (Figure 7.7).
A cation's ionic potential - a property correlated
with the element's electronegativity - is a useful guide
to its behaviour in molten and crystalline silicates
(Figure 9.1.1) and in aqueous solution (Figure 9.3.1).
Bonding in silicates
One particularly important bond in view of its role in
silicate structure (Chapter 8) is the Si-O bond, which
appears from Figure 7.8a to possess around 50% ionic
character and therefore to combine ionic and covalent
bonding in more or less equal amounts. The small
nominal ionic radius of the Si
4+
'cation' (34 pm Box 7.2)
is consistent with its occupation of tetrahedral sites in
all silicate minerals. In view of its high charge and
small radius, however, the Si
4+
cation must be highly
polarizing and its distortion of the oxygen ion intro-
duces an significant degree of covalency into the Si-O
bond. The concept of an Si
4+
cation is therefore an
approximation to be used with caution: the charge
residing on a silicon atom in a real silicate structure
actually approximates to 2+ rather than 4+. Silicon, like
carbon, uses an sp
3
-hybrid in forming covalent bonds,
and its tetrahedral co-ordination is therefore as much a
reflection of covalent bonding between Si and O as of
ionic bonding. The relative covalency of the Si-O bond
accounts for the structural coherency of the chain,
sheet and framework skeletons of many silicate miner-
als (Chapter 8). Like phosphorus and sulfur, Si shows
little tendency to form double or
π
-bonds. Unlike C, it
never forms an sp
2
-hybrid.
The other chemical bonds operating in silicates are all
more ionic in character than the Si-O bond. Al
3+
is a
small ion with a relatively high charge, but its ionic
potential is barely more than half that of Si
4+
(Figure 7.7)
and the Al-O bond is regarded as being nearly 60%
ionic (Figure 7.8a). Mg-O is about 65% ionic and Ca-O,
Na-O and K-O are all more than 75% ionic. For these
elements the ionic model provides the most appropriate
predictor of co-ordination and crystal structure. The
Polarization of a covalent bond: ionicity
In an ideal covalent bond, two valence electrons are
shared equally between two atoms. The two atoms
must have equal power to attract electron density to
keep it positioned symmetrically between them. If
they differ even slightly in electronegativity, electron
density will be gathered more around the more elec-
tronegative atom, giving it a slight negative net charge
(and leaving a complementary positive charge on the
other atom). Such 'polarization' of the covalent bond is
equivalent to transferring a fraction of an electron from
one atom to the other, thus introducing a degree of
ionic character or 'ionicity' into the bond.
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