p17p35ujimn9ejc9cap81i1j7e4.pdf

D GENERALIZATIONS ABOUT CRYSTAL STRUCTURES BASED ON PERIODICITY
p
s
sp 3
lone pair
Figure 1.12. The hybridization of the inequivalent s and p orbitals of group V and VI atoms
leads to four sp 3 orbitals that have a tetrahedral arrangement, even though some of the
ligands are nonbonding lone pairs. The shaded circles represent neighboring atom positions
and the dots represent electrons in lone pair orbitals. The bond angles of the actual ligands
usually vary from the ideal.
neighbors and two H nearest neighbors. While the bonding geometry is not perfectly
tetrahedral, the C-C-C bond angle is 120°, so that the C backbone forms
a zig-zag chain on the atomic scale. Pseudo-tetrahedral arrangements are found
even when there are fewer than four nearest neighbors. In these cases, the lone
pairs of electrons complete the tetrahedron, as illustrated in Fig. 1.12.
As a final example of the importance of orbital hybridization, we consider
the silicates. Silicates are a wide class of silicon and oxygen containing compounds
that make up much of the earth’s crust. Substances such as quartz,
micas, clays, and zeolites, which are commonly thought of as minerals, are also
important engineered materials that can be used for piezoelectrics (quartz) and
hydrocarbon cracking catalysts (clays and zeolites). The basic structural unit of
a silicate is the SiO 4 tetrahedron which links in complex patterns to form a wide
variety of structures. The tetrahedral coordination of the Si atom by O is a result
of orbital hybridization. Furthermore, these tetrahedral SiO 4 units always link
by corners, placing the O atom in two-fold coordination (in this case, O has two
electropositive ligands and two nonbonding lone pairs). Figure 1.13 shows the
atomic arrangement found in cristobalite, a representative silicate structure.
In conclusion, there are three established generalizations which relate the
p
s
sp 3
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