handbook of carbon, graphite, diamond and fullerenes

16 Carbon, Graphite, Diamond, and Fullerenes
case of the carbon atom, the valence electrons are the two 2p orbitals.
Carbon in this state would then be divalent, since only these two electrons
are available for bonding.
Divalent carbon does indeed exist and is found in some highly reactive
transient-organic intermediates such as the carbenes (for instance methylene).
However, the carbon allotropes and the stable carbon compounds are
not divalent but tetravalent, which means that four valence electrons are
present. 16 ' Howthis increase in valence electrons occurs is reviewed in Sec.
3.0.
The carbon valence electrons are relatively easily removed from the
carbon atom. This occurs when an electric potential is applied which
accelerates the valence electron to a level of kinetic energy (and corresponding
momentum) which is enough to offset the binding energy of this
electron to the atom. When this happens, the carbon atom becomes ionized
forming a positive ion (cation). The measure of this binding energy is the
ionization potential, the first ionization potential being the energy necessary
to remove the first outer electron, the second ionization potential, the
second outer second electron, etc. The ionization energy is the product of
the elementary charge (expressed in volts) and the ionization potential,
expressed in electron volts, eV (one eV being the unit of energy accumulated
by a particle with one unit of electrical charge while passing though a
potential difference of one volt).
The first ionization potentials of carbon and other atoms close to
carbon in the Periodic Table are listed in Table 2.2. It should be noted that
the ionization energy gradually (but not evenly) increases going from the
first element of a given shell to the last. For instance, the value for lithium
is 5.39 V and for neon, 21.56 V. It is difficult to ionize an atom with a
complete shell such as neon, but easy to ionize one with a single-electron
shell such as lithium.
As shown in Table 2.2 above, carbon is located half-way between the
two noble gases, helium and neon. When forming a compound, carbon can
either lose electrons and move toward the helium configuration (which it
does when reacting with oxygen to form CO2), or it can gain electrons and
move toward the neon configuration (which it does when combining with
other carbon atoms to form diamond).
The six ionization potentials of the carbon atom are shown in Table 2.3.