At the heart of chemistry lies the periodic system of chemical elements. Despite being the cornerstone of modern chemistry, the overall structure of the periodic system has never been fully understood from an atomic physics point of view. Group-theoretical models have been proposed instead, but they suffer from several limitations. Among others, the identification of the correct symmetry group and its decomposition into subgroups has remained a problem to this day. In an effort to deepen our limited understanding of the (...) periodic law, we have extended the traditional Lie algebraic framework to account for the peculiar degeneracy structure of the periodic system. Starting from the four-dimensional hidden symmetry and accidental degeneracy of the hydrogen atom, as first revealed by Fock in 1935, our research has mainly focussed on the way this SO(4) symmetry of the Coulomb potential gets broken in the periodic system as a consequence of the transformation of the hydrogenic (n, l) filling order to the Madelung (n+l, n) order due to electronic repulsions, relativistic effects and spin-orbit couplings. In this PhD dissertation, a new left-step format of the periodic table is first proposed on the basis of the Madelung rule. Following the particle physics tradition, the chemical elements are then considered as various states of some 'atomic matter', which is described by a non-compact spectrum-generating dynamical Lie group. The chemical elements are shown to form a basis for a single infinite-dimensional degeneracy space of the SO(4,2) ⊗ SU(2) group. An explanation for the period doubling is then proposed in terms of a particular symmetry breaking of the SO(4,2) group to the anti de Sitter SO(3,2) group. The Madelung rule is rationalised on the basis of nonlinear Lie algebras which reflect the screening of the Coulomb hole. This opens new perspectives for a symmetry-based understanding of how the periodic law emerges from its quantum mechanical foundations, and holds the future promise of complementing our current phenomenological approach by a direct atomic physics approach. (shrink)
This chapter gives an overview of the evolution of the position of the rare-earth elements in the periodic system, from Mendeleev’s time to the present. Three fundamentally different accommodation methodologies have been proposed over the years. Mendeleev considered the rare-earth elements as homologues of the other elements. Other chemists looked upon the rare earths as forming a special intraperiodic group and therefore clustered the rare-earth elements in one of the groups of the periodic table. Still others adhered to the intergroup (...) accommodation of the rare earths, according to which the rare-earth elements do not show any relationship with the other elements, so that they had to be placed within the periodic table as a separate family of elements. The intergroup accommodation became the preferred one in the twentieth century. The advantages and disadvantages of the different representations of the modern periodic table are discussed. (shrink)
Since Mendeleev’s discovery in 1869, the periodic table has figured as the ultimate paper tool in chemical research. It has proved to be a vital research instrument in the arsenal of the chemical community. No chemistry textbook, lecture theatre or scientific laboratory is complete without a copy of the periodic table of the elements. -/- This however, should not necessarily imply that the periodic table has never had to contend with problems. In this thesis, the history of the accommodation of (...) the rare-earth elements in the periodic table will be addressed. When Mendeleev published his periodic table in 1869, the rare earths already constituted a major obstacle. Mendeleev was able to include only four members of the rare earths and he experienced great difficulties in positioning these elements. Question marks and wrong atomic weights reigned in the last rows of Mendeleev’s system. -/- This problematic accommodation quickly grew into one of the most serious threats for the periodic law. For over fifty years, chemists continually struggled with the placement of these maddeningly similar elements. As a consequence, a lot of chemists started to question the validity of the periodic law, but others took it as a sign that the concept of a chemical element had to be reconsidered. As a result, this work intends to retrace the mutual influence of the philosophical ideas about the nature of chemical elements and the development of the periodic table from its inception in 1869 to the discovery of Moseley’s law in 1913 and Bohr’s publication of his landmark paper On the Constitution of Atoms and Molecules. -/- The aim of this thesis is to show how, on the one hand, the periodic table (as a research instrument) helped in reformulating the contemporary ideas about chemical elements, and how it aided in developing a new research program in order to resolve the rare earth crisis. On the other hand, the question will be taken up to what extent Crookes’ evolutionary ideas about meta elements helped in saving the periodic table from a severe downfall by solving the gnawing problem of the rare-earth elements. In particular, this work will also focus on the investigations of the Czech chemist, Bohuslav Brauner. It will be shown how Brauner, under the influence of Crookes’ ideas, was led to his formulation of the Asteroid Hypothesis, according to which all the rare-earth elements should be placed in a single case of the periodic table. (shrink)
The Periodic Table has the column of the noble gas atoms (He, Ne, Ar, Kr, Xe, Rn) as one of its main pillars. Indeed the inert chemical nature of their closed shell structure is so striking that it is sometimes extended to all such structures. Is it true however that any closed shell, say a closed d-subshell will denote a lack of chemical activity? Take the noble metals for instance, so renowned for their catalytic capacity. Platinum has 10 electrons in (...) its valence shell which makes one of its excited states to be a closed 5d10–6s0 state. Surely this state would not be expected to be crucial to the catalytic activity of platinum, or would it? Or take palladium whose ground state is precisely the 4d10–5s0 state, should we expect that an isolated Pd atom at near zero-point temperature would attack a closed-shell hydrogen molecule efficiently? We shall here show that this is precisely the case; the closed-shell excited states of nickel and platinum are indeed crucial, through symmetry avoided crossings, for their reactivity. Other valuable catalysts as ruthenium depend on their excited states with maximal d-shell occupancy for their activity. The most notable confirmation of this new finding; that closed d-shells are vital to the catalytic activity of noble metals however, is the case of palladium whose closed-shell ground state is indeed capable of attacking hydrogen and hydrocarbon molecules even at temperatures well below 10 K as was predicted theoretically and immediately confirmed experimentally. (shrink)
What are the criteria determining the individuation of chemical kinds? Recent philosophical discussion, which puts too much emphasis on microstructure, seems to presuppose a reductionist conception not motivated by the scientific facts. The present article traces the development of the traditional notion of a substance with the rise of modern chemistry from the end of the 18th century with a view to correcting this speculative distortion.