The physical meaning of the bonding edges of the graph was however unknown.
Nevertheless, it rapidly appeared to Berzélius that electric rather than caloric forces
were accountable for the bonding [8], this idea was further reformulated by Laming
[9] in a fully atomistic fashion accounting for Faraday’s electrochemical equivalent.
Laming’s hypotheses anticipate the atomic electronic shell structure half a century
before Joseph John Thomson’s discovery of the electron:
a mass of electrical matter, or electricity, may be regarded as composed of electrical atoms,
just as a mass of ordinary matter contains ordinary atoms; and thus the sphere of electricity
which surrounds an ordinary atom will consist of a number of electrical atoms arranged in
concentric strata. The number of electrical atoms belonging to a given ordinary atom may
be assumed to be such as to complete its external spherical stratum, or, on the contrary, it
may be such as to leave that external spherical stratum more or less imperfect.
A few years after Thomson’s discovery of the electron, G. N. Lewis proposed in
a memorandum dated March 28, 1902 [7], his cubic atomic model in which the
vertices are occupied or not by electrons according to the element’s column in the
Periodic Table. In this way, he established a direct link between electrons and the
concept of valence which provides a foundation to Abegg’s valence and countervalence law [10]. It is worth noting that these different atomic models have been
conceived on the only basis of chemical arguments. Moreover, Lewis atom is closer
to nowadays representations than Thomson’s 1904 plum-pudding model [11].
Lewis’s atom in molecule is composed of a kernel grouping the nucleus and the
inner shell electrons and an outer shell, the valence shell. The atom tends to have an
even number of electrons in its valence shell and especially eight electrons which
are symmetrically arranged at the corner of a cube. The atomic shells of two bonded
atoms mutually interpenetrate and therefore electrons may belong to the valence
shells of two bonded atoms. Lewis emphasized the concept of electron pair as the
cornerstone of molecular structure and proposed to write the formulas of chemical
compounds by using atomic symbols surrounded by a number of dots corresponding to the number of electrons in the atomic shell. In spite of its simplicity,
Lewis’ approach is remarkably efficient and remains fundamental for basic chemical education. Although Lewis’ model explains the structure of a majority of
molecular species, it fails, for example, to account for the hexagonal structure of
benzene or for the paramagnetism of dioxygen. Whereas Huggins’ attempt to
understand benzene by a single Lewis structure yielded chimerical representations
[12], the concept of mesomery, pioneered by Ingold [13, 14], which considers a
weighted superposition of structures has been very successful with this respect and
therefore constitutes an important complement to Lewis’s model. In order to be able
to treat dioxygen, Linnett modified the original Lewis model by splitting the initial
octet into two sets of four electrons, one having one spin quantum number and the
other the opposite value [15, 16].
Until this point, a molecule has been formally described from a set of N nodes
(nuclei) by three features only: a labelling (composition: stoichiometry, bruto formula), a topology (connection: bonding skeleton), and a topography (constitution:
Lewis structures, resulting from the application of the labelling on the topology).
1 Topological Approaches of the Bonding in Conceptual Chemistry
3
Nevertheless, it rapidly appeared to Berzélius that electric rather than caloric forces
were accountable for the bonding [8], this idea was further reformulated by Laming
[9] in a fully atomistic fashion accounting for Faraday’s electrochemical equivalent.
Laming’s hypotheses anticipate the atomic electronic shell structure half a century
before Joseph John Thomson’s discovery of the electron:
a mass of electrical matter, or electricity, may be regarded as composed of electrical atoms,
just as a mass of ordinary matter contains ordinary atoms; and thus the sphere of electricity
which surrounds an ordinary atom will consist of a number of electrical atoms arranged in
concentric strata. The number of electrical atoms belonging to a given ordinary atom may
be assumed to be such as to complete its external spherical stratum, or, on the contrary, it
may be such as to leave that external spherical stratum more or less imperfect.
A few years after Thomson’s discovery of the electron, G. N. Lewis proposed in
a memorandum dated March 28, 1902 [7], his cubic atomic model in which the
vertices are occupied or not by electrons according to the element’s column in the
Periodic Table. In this way, he established a direct link between electrons and the
concept of valence which provides a foundation to Abegg’s valence and countervalence law [10]. It is worth noting that these different atomic models have been
conceived on the only basis of chemical arguments. Moreover, Lewis atom is closer
to nowadays representations than Thomson’s 1904 plum-pudding model [11].
Lewis’s atom in molecule is composed of a kernel grouping the nucleus and the
inner shell electrons and an outer shell, the valence shell. The atom tends to have an
even number of electrons in its valence shell and especially eight electrons which
are symmetrically arranged at the corner of a cube. The atomic shells of two bonded
atoms mutually interpenetrate and therefore electrons may belong to the valence
shells of two bonded atoms. Lewis emphasized the concept of electron pair as the
cornerstone of molecular structure and proposed to write the formulas of chemical
compounds by using atomic symbols surrounded by a number of dots corresponding to the number of electrons in the atomic shell. In spite of its simplicity,
Lewis’ approach is remarkably efficient and remains fundamental for basic chemical education. Although Lewis’ model explains the structure of a majority of
molecular species, it fails, for example, to account for the hexagonal structure of
benzene or for the paramagnetism of dioxygen. Whereas Huggins’ attempt to
understand benzene by a single Lewis structure yielded chimerical representations
[12], the concept of mesomery, pioneered by Ingold [13, 14], which considers a
weighted superposition of structures has been very successful with this respect and
therefore constitutes an important complement to Lewis’s model. In order to be able
to treat dioxygen, Linnett modified the original Lewis model by splitting the initial
octet into two sets of four electrons, one having one spin quantum number and the
other the opposite value [15, 16].
Until this point, a molecule has been formally described from a set of N nodes
(nuclei) by three features only: a labelling (composition: stoichiometry, bruto formula), a topology (connection: bonding skeleton), and a topography (constitution:
Lewis structures, resulting from the application of the labelling on the topology).
1 Topological Approaches of the Bonding in Conceptual Chemistry
3