crystallographers. The Braggs’ elucidation of the structures of diamond and graphite
some years later led to the first structural confirmation of allotropy. A phenomenon is
recognised by Berzelius in the previous century [37]. Structural determinations in a
number of laboratories in Europe, USA and Japan proceeded at a rapid rate until
1914, when the outbreak of the First World War required many young scientists to
participate in their country’s war efforts. Henry Moseley and William Henry Bragg’s
elder son sadly lost their lives whilst on active service. The structural studies of
simple salts led to a better understanding of how the sizes of the atoms could
influence the structures of salts and minerals. V.M. Goldschmidt was the first to
tabulate covalent and ionic radii and begin to develop the structural principles of
inorganic solid-state chemistry and lay the foundations of geochemistry [44].
In the 1916 papers, published independently by Kossel and Lewis, the fundamental descriptions of the chemical bond were defined [45–47]. They qualitatively
described for the first time the dual extremes of chemical bonding by introducing
ionic and covalent bonding types and suggested that the number of outer electrons of
an atom played a very important role in determining the valencies and oxidation
states of the elements [48]. Their ideas are still recognisable in current introductory
undergraduate courses in chemistry. This duality was based on either the transfer or
sharing of electrons to enable each atom to achieve an octet of electrons. Lewis’
non-quantum mechanical description and his classical formulation in terms of
localised electrons occupying the vertices of cubes reflected the influence of X-ray
diffraction studies, and Kossel’s description of electrons occupying circular orbitals
owed much to Bohr’s model for the hydrogen atom. Their descriptions gained
credence from the X-ray crystallographic structures which were determined during
this period.
The early structures utilised the classical knowledge of crystallography to assist
the interpretation of the diffraction patterns, and the high symmetry of the crystals
meant that the structures had few variable parameters. These early studies concentrated on well-formed crystals with high symmetries which could be solved readily
once the cell dimensions and the symmetry of the unit cell were determined.
Therefore, there was a concentration on inorganic crystals which established that
the salts had very symmetric infinite structures. This gave strong support for the
ionic description of chemical bonding proposed by Kossel. Support for Lewis’
covalent bonding description was obtained in the structural determinations of graphite and diamond and those salts which had molecular anions, e.g. CO 3
2À and NO 3
À
salts (see Table 3). The first structure of a simple organic molecule, hexamethylenetetramine, was completed in 1923 and showed it had the high-symmetry
adamantane-related structure shown in Fig. 8. It crystallises in a body-centred
cubic lattice which means that there are few variables associated with solving the
structure and the high symmetry of the space group places each atom in a highsymmetry site. The lack of strong scattering associated with the hydrogen atoms
meant that the tetrahedral geometries at carbon had to be implied rather than
determined, and it was not until neutron diffraction was established after World
War 2 that the positions of the hydrogen atoms were accurately located.
20
D. M. P. Mingos
some years later led to the first structural confirmation of allotropy. A phenomenon is
recognised by Berzelius in the previous century [37]. Structural determinations in a
number of laboratories in Europe, USA and Japan proceeded at a rapid rate until
1914, when the outbreak of the First World War required many young scientists to
participate in their country’s war efforts. Henry Moseley and William Henry Bragg’s
elder son sadly lost their lives whilst on active service. The structural studies of
simple salts led to a better understanding of how the sizes of the atoms could
influence the structures of salts and minerals. V.M. Goldschmidt was the first to
tabulate covalent and ionic radii and begin to develop the structural principles of
inorganic solid-state chemistry and lay the foundations of geochemistry [44].
In the 1916 papers, published independently by Kossel and Lewis, the fundamental descriptions of the chemical bond were defined [45–47]. They qualitatively
described for the first time the dual extremes of chemical bonding by introducing
ionic and covalent bonding types and suggested that the number of outer electrons of
an atom played a very important role in determining the valencies and oxidation
states of the elements [48]. Their ideas are still recognisable in current introductory
undergraduate courses in chemistry. This duality was based on either the transfer or
sharing of electrons to enable each atom to achieve an octet of electrons. Lewis’
non-quantum mechanical description and his classical formulation in terms of
localised electrons occupying the vertices of cubes reflected the influence of X-ray
diffraction studies, and Kossel’s description of electrons occupying circular orbitals
owed much to Bohr’s model for the hydrogen atom. Their descriptions gained
credence from the X-ray crystallographic structures which were determined during
this period.
The early structures utilised the classical knowledge of crystallography to assist
the interpretation of the diffraction patterns, and the high symmetry of the crystals
meant that the structures had few variable parameters. These early studies concentrated on well-formed crystals with high symmetries which could be solved readily
once the cell dimensions and the symmetry of the unit cell were determined.
Therefore, there was a concentration on inorganic crystals which established that
the salts had very symmetric infinite structures. This gave strong support for the
ionic description of chemical bonding proposed by Kossel. Support for Lewis’
covalent bonding description was obtained in the structural determinations of graphite and diamond and those salts which had molecular anions, e.g. CO 3
2À and NO 3
À
salts (see Table 3). The first structure of a simple organic molecule, hexamethylenetetramine, was completed in 1923 and showed it had the high-symmetry
adamantane-related structure shown in Fig. 8. It crystallises in a body-centred
cubic lattice which means that there are few variables associated with solving the
structure and the high symmetry of the space group places each atom in a highsymmetry site. The lack of strong scattering associated with the hydrogen atoms
meant that the tetrahedral geometries at carbon had to be implied rather than
determined, and it was not until neutron diffraction was established after World
War 2 that the positions of the hydrogen atoms were accurately located.
20
D. M. P. Mingos