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Periodic Table (Chemistry) 685

FIGURE 7 Schematic diagrams of the three types of bonding mechanisms: T, I, and E. Each arrow represents a
one-electron “hop.” Note that the I-bond promotes aggregation because a third party is needed at the head of one
arrow. Two arrows count for one bond.

the Chemical Code: Bonding Across the Periodic Table, element. “Execution” of one, two, etc. arrows generates
it is suggested that two arrows affixed on two singlet- a unique ionic configuration. The allocation of arrows is
coupled electrons constitute one bond. Superior bonds are made according to the association rule:
formed when arrows are oriented in a codirectional (head
to tail, HT) fashion. Two HT arrows can form either a 1. One arrow symbolizes the transfer of one electron
closed or an open loop. The former corresponds to a T- (1 − e hop), and it causes kinetic energy reduction
bond promoting segregation of electron pairs, while the proportional to overlap.
latter corresponds to an I-bond promoting aggregation be- 2. A dot can have a maximum of one entering (inbound)
cause a third party is needed at the head (tail) of one arrow, and one exiting (outbound) arrow.
as illustrated in Fig. 7. 3. A pair can have a maximum of two exiting arrows.
At the limit of the hydrogen molecule, the T-bond is 4. A hole can have a maximum of two entering arrows.
equivalent to the covalent bond of VB theory, i.e., the 5. Outbound arrows are called diastolic, and inbound
interaction of two diradical and two ionic VB configu- arrows are called systolic.
rations. By contrast, the concept of the T-bond leads to
molecular formulae which are called T-formulae. These The resulting molecular formula is called the arrow
are fundamentally different from Lewis formulae as they formula, and it displays pairs, dots, and holes connected
constitute a representation of the molecule which reveals by affixed arrows. Execution of one arrow, two arrows
not only stoichiometry and shape but also intrinsic stabil- in all possible combinations, three arrows in all possible
ity. At the limit of a metal solid, the I-bond can be said to combinations, and so on generates an ensemble of VB
be equivalent to the “metallic bond,” although the last term configurations, the interactions of which are primarily re-
is equivocally defined. Finally, the E-bond can be taken sponsible for binding. On the basis of energetic consid-
to be analogous to the ionic bond provided that induction erations, recognizing that two arrows acting concertedly
and dispersion are also included in this definition. In sum- (i.e., representing a single 2-e hop) achieve the same goal
mary, the new idea is that instead of classifying molecular as the same two operating sequentially (i.e., representing
experimental data according to the covalent/ionic gospel, two ordered 1 − e hops), and that 1 − e and 2 − e hops in-
one is now offered three receptacles in which to enter his teract, we focus attention on the possible 2 − e hops. Here,
observations: T, I, and E. This marks a new beginning for we recognize two variants: an exchange 2 − e hop which
conceptual theoretical chemistry. conserves charge and counts as one T-bond, and a charge
transfer(CT)2 − ehopthatengenderseitherinter-orintra-
atomic charge separation and counts as one I-bond. Note
A. The Three Types of Bonds: T, I, and E
that in each case two arrows are taken together to signify
The recipe for the chemical implementation of VB theory either a T- or an I-bond. Since the number of total arrows
is very simple: a molecule is made up of atoms, each in the within the formula is conserved, we have a fixed number
electronic configuration deemed appropriate for bonding. of pairwise arrow combinations and, hence, a fixed num-
Each atomic electronic configuration has any one or all of ber of bonds (recall that two arrows count as one bond).
three elements: doubly occupied atomic orbitals (AOs), Starting now from the arrow formula and specifying the
called “pairs”; singly occupied AOs, called “dots”; and pairwise arrow combinations (pictorially or symbolically)
vacant AOs, called “holes.” The maximum electron de- produces the formula called the VB descriptor. The linear
localization which can be sustained by each one of the combination of VB descriptors (each one corresponding
three elements, and which is consistent with the Pauli Ex- to a linear combination of VB configurations) produces
clusion Principle, is depicted by affixing arrows on each the operationally significant total VB wave function, a