Geology Reference
In-Depth Information
(a)
(b)
(c)
142 pm
sp
2
hybrid (
σ
-bonding)
p orbital (
π
-bonding)
Figure 7.4.1
Different views of bonding in graphite. (a) Disposition of the sp
2
hybrid and p orbital lobes. (b) Intra-layer
and interlayer bond lengths. (c) the three alternative ways of allocating the intra-layer double bonds in classical bonding
theory; in the wave-mechanical view, the
π
-bonds are delocalized throughout the layer.
attraction called the
van der Waals interaction
, described
later in the chapter, which is too weak to prevent sheets
slipping easily over each other. the extreme softness of
graphite and the large interlayer spacing (335 pm) indicate
how feeble the interlayer force is. as there is no intercom-
munication of electron orbitals between sheets, graphite
acts as an insulator in directions perpendicular to the
sheets.
Noting the dramatic contrast between the extremely
strong intra-layer bonding in graphite and the feeble van
der Waals interaction binding one layer to the next, it is
natural to ask whether individual layers could actually exist
on their own. the answer - discovered only in the last
decade - is 'yes'. Such monolayers constitute the novel
form of carbon now known as
graphene
, one of several
newly discovered forms of carbon discussed in Box 9.6.
Box 7.5 Lewis acids and bases
In 1923 the american chemist G.N. Lewis widened the
concept of acids and bases to encompass systems in
which h
+
ions (upon which the traditional notion of acids
and bases is founded - appendix B) are not available,
such as in silicate melts. Lewis' ideas also have partic-
ular relevance to co-ordination complexes, in which
bonding involves the sharing of a 'lone pair' of electrons
donated by one of the participating atoms. Lewis defined
a base as 'an atom or molecule capable of donating an
electron pair to a bond' whereas a 'Lewis acid' is an
atom or molecule that can accept a lone pair. the Lewis
definition of 'acid' embraces the traditional 'h
+
donor'
viewpoint (since h
+
can readily attach to a lone pair,
forming a covalent bond) but has much wider
application.
how can we apply the Lewis concept to co-ordination com-
plexes? as we saw in Chapter 4, the co-ordination complex
Cu(hS)
3
2-
is believed to play an important part in the low-
temperature hydrothermal transport of Cu. the electron con-
figuration of the Cu
+
ion is [ar] 4s
0
3d
10
4p
0
. the complex
forms because a lone pair of electrons on each hS
-
ion over-
laps with a vacant Cu orbital (4 s or 4p), forming a molecular
orbital that allows the electron pair to be associated with two
atoms (S and Cu). here each hS
-
ion is acting as a Lewis
base (electron-pair donor) and the Cu
+
ion is the Lewis acid.
the Lewis approach provides valuable insights into the
chemistry of silicate melts, and helps us to understand
why, for example, some metals prefer to be associated
with sulfide minerals whereas others have more affinity
with silicates (Box 9.8).
Search WWH ::
Custom Search