Periodic table, atomic theoretical explanation

In order to be able to derive deductively the construction of the periodic table, one must be able to deduce theoretically the maximum number of electrons which can occupy orbits of the same nk. This can now be done consideration of physical3 and chemical4 experimental evidence first suggested the rule that the maximum number of electrons which can occupy equivalent wfc-orbits in a single atom is 2(2 —1) a theoretical explanation of this rule can now be given, but it lies outside the scope of this book. 

In 1922, the year his Nobel Prize made him a Danish national hero, Bohr accomplished a second great theoretical triumph an explanation of the atomic structure that underlies the regularities of the periodic table of the elements. It linked chemistry irrevocably to physics and is now standard in every basic chemistry text. Around the nucleus, Bohr proposed, atoms are built up of successive orbital shells of electrons—imagine a set of... 

Pauli, Wolfgang (1900-58) Austrian-born Swiss theoretical physicist. Pauli is best known for his enunciation of the Pauli exclusion principle in 1925. This enabled the electronic structure of atoms to be understood, particularly how the shell structure of atoms, and hence the periodic table of the elements, comes about. Pauli won the 1945 Nobel Prize for physics for this work. Pauli made many other important contributions to physics including his prediction of the neutrino in beta decay, the incorporation of spin into quantum mechanics, and the explanation of paramagnetism in metals. He also wrote several classic books and reviews on quantum mechanics. 

In previous chapters we have seen how Dalton s atomic theory encountered difficulties, especially during the first half of the nineteenth century. After 1860 the chemical atom became so useful, especially to organic chemists, that only a few dissenting voices were heard in this branch of the science. However, to many physicists the atomic concept seemed unnecessary, and some of the great advances in nineteenth century physics had been made without reference to atomism. Then, in a series of discoveries which were made in rapid succession around the turn of the century, the existence of atoms was established to the satisfaction of everyone. All atoms were shown to contain identical subatomic particles, which were called electrons. Furthermore, certain atoms were shown to be undergoing spontaneous and continuous transmutation into others. The atomic concept may have been vindicated, but at the cost of disproving Dalton s tenets of the indivisibility and immutability of atoms. Further advances placed the periodic table on a firm theoretical basis, and provided an explanation of the forces involved in chemical bonding. 

Core-valence correlation involves the interaction between the inner shell (core) and valence electrons. That this interaction is small is an important axiom of chemistry, as it is well established that the properties of atoms and molecules are largely determined by the valence electrons. This principle underlies the explanation of chemical periodicity and the structure of the periodic table. Conceptually, one considers the inner electrons to be tightly bound and rather inert. Hence, most theoretical studies only consider valence electron correlation with the core electrons frozen at the Hartree-Fock (HF) or multi-configuration self-consistent field (MCSCF) level or replaced with a pseudopotential. The utility and accuracy of the vast body of quantum chemical calculations provide ample evidence justifying this assumption. 

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