CHEMICAL BONDING AND THE PROPERTIES OF MINERALS - Chemical Fundamentals of Geology and Environmental Geoscience - page 122

Box 7.4 Case study: bonding in graphite and graphene

Graphite is a soft, greasy, black, lustrous mineral, widely
used as a lubricant, as an electrical conductor, and of
course as pencil 'lead' (the name 'graphite' derives from
the Greek verb 'to write'). It is impossible to imagine a
greater contrast between these properties and those of
the other carbon mineral, diamond. Why are these two
forms of carbon so different?
Figure 7.4.1b shows that graphite, unlike diamond, is
made up of sheets. each sheet is a continuous network of
interconnected hexagonal rings, in which every carbon
atom is bonded to three equidistant neighbours, the bonds
sticking out symmetrically at 120° to each other. this 'trig-
onal planar' bonding geometry (Figure 7.4.1a) is the signa-
ture of the sp 2 -hybrid, formed by combining the 2 s orbital
of carbon with two of the three 2p orbitals. the three co-
planar sp 2 lobes establish σ -bonds with neighbouring
atoms.
the C-C bond length within the graphite sheets is
142 pm, a little shorter than the C-C distance in diamond
(154 pm). as stronger bonds pull atoms closer, this sug-
gests that the intra-sheet bonding in graphite is actually
stronger than the bonding in diamond, the hardest sub-
stance known. the explanation lies in the fourth unpaired
electron of carbon, excluded from the sp 2 -hybrid
(Figure 7.4.1a). this occupies a p-orbital projecting perp-
endicular to the sheet, and it allows π -bonds to be formed

laterally between neighbouring atoms, reinforcing the intra-
sheet σ -bond network.
In classical bonding theory, this π -bond would be added
to one of the three σ -bonds, transforming it into a double
bond, with the other two bonds remaining single.
Figure 7.4.1c shows there are three alternative ways of
distributing these double bonds throughout the sheet so
that every atom is involved in four bonds. In the wave-
mechanical interpretation, however, the real situation is a
combination of all three configurations: the p-electron in
each atom can be involved in partial π -bonds with all three
neighbours at once. Consequently the π -bond 'sausages'
of electron density above and below the plane (Figure 7.4c)
are not localized between two specific atoms but are
spread thinly all round the hexagon and indeed throughout
the sheet. the π -electrons are thus delocalized in intercon-
necting molecular orbitals, resembling sheets of chicken-
wire, within which the energy structure is like that of a
metal (Box 7.6), allowing the π -electrons to migrate across
the entire sheet if an electric field is applied. thus graph-
ite is a good electrical conductor in directions parallel to
the sheets. this conducting behaviour is also responsible
for the opaque, metallic appearance of graphite.
Intra-layer bonding in graphite ties up all four valence
units of carbon. the adhesion of one sheet to another
(interlayer bonding) is achieved through a much weaker

carbon atom forms with its four neighbours. The same
structure is found in metallic silicon, also a fairly hard
material. sp 3 geometry is characteristic of single-
bonded carbon compounds, like the saturated hydro-
carbons (Chapter 9). When carbon forms a double
bond, however, it utilizes an alternative sp 2 -hybrid . This
has a different geometry, found in the mineral graphite
(Box 7.4) and in the benzene molecule (C 6 H 6 ), the basic
unit of aromatic carbon compounds (Chapter 9).

of valence electrons. By overlapping with an empty
orbital in the metal atom, each ligand lone pair forms a
bonding molecular orbital which can accommodate
the two electrons at a lower energy. This co-ordinate
bond 2 is a variety of covalent bond in which both elec-
trons are supplied by one of the participating atoms
(the ligand) instead of one electron being supplied by
each of them. Such complexes are stable because each
electron, enjoying the attraction of two nuclei (cf.
Box 7.3), has a lower energy than in the isolated atomic
orbital. Current chemical thinking views the formation
of a co-ordinate bond as a particular type of acid-base
reaction (Box 7.5).
Co-ordination complexes are important in geochem-
istry because they markedly increase the solubility of

The co-ordinate bond

In some circumstances a covalent-like bond can also be
established using a lone pair. This arises in co-
ordination complexes, which consist of a central metal
atom or ion (commonly a transition metal) surrounded
by a group of ligands , electronegative ions or small
molecules (such as NH 3 or HS - ) that possess lone pairs

Not to be confused with the co - ordination polyhedron of an
ionic compound.

2