ABSTRACT
Organic molecules are based on carbon atoms by definition. The
unique complexity of organic chemistry is related to two factors.
First, carbon atoms are predisposed to forming chemical bonds
among themselves rather thanwith other atoms. Like other group IV
elements, carbon has intermediate electron affinity. This is in sharp
contrast to, for example, alkaline atoms’ low electron affinity, which
tends to react with the high electron affinity of halogen atoms on
the opposite side of the periodical table. Second, the carbon-carbon
bond has a particularly high strength within group IV because of
the carbon atom’s low atomic number and small atomic size. Indeed
the carbon-carbon bond energy is about 1.5 times higher than the
silicon-silicon bond energy [1]. The carbon atom has six electrons in
total, with two in the inner helium core and four valence electrons in
the outer 2s and 2p orbitals. There are two types of common carbon
bonds corresponding with the different hybridizations of 2s and
2p orbitals. In sp3 hybridization, one spherical 2s orbital and three
2p orbitals form four superposition wavefunctions pointing to the
vertices of a tetrahedron. In a diamond, each carbon atom forms four
sp3 bonds with four neighboring carbon atoms in the tetrahedral
structure. Diamond has a high energy bandgap and absorbs no
visible light. From a chemical point of view, both of the bonding
carbon atoms donate one valence electron to the sp3 bond. Another
common type of bond is the sp2 hybridizationwhere the spherical 2s
orbital forms superposition with 2px and 2py orbitals. All the three
orbitals are symmetrical functions respective to the z-axis. In other words, they do not change sign as the coordinate z is changed to –z. The three hybridized orbitals point to the vertices of a triangle in
x-y plane with a bond angle of 120◦. In graphite, the carbon atoms have sp2 bonds and form a honeycomb lattice, as shown in Fig. 1.1.
