Quantum mechanics outer shell configurations

Although Pauli s principle was rescued by the new quantum theory, the notion of individual quantum numbers for each electron was lost. The concept of electronic configurations cannot be derived from quantum mechanics. It represents an approximation and a bookkeeping scheme for finding the number of outer electrons in an atom, but does not necessarily provide information as to the inner electron shells (Scerri, 1991). On the superposition principle, see Amann (1990) and Woolley (1991). 

Elements in the same group of the periodic table possess the same number of electrons in their outer shells and are therefore said to have the same valence electronic configuration, and consequently similar chemical and physical properties. As electrons are filled into the inner shells of an atom, the outer shell takes on a specific valence configuration that is determined by the rules that govern how many electrons can occupy a particular shell, as described above in the section on quantum mechanics. It is this very regularity in the number of outer-shell electrons that explains the periodic behavior shown by the elements as the atomic number increases. Similarly, properties such as atomic size are determined by the number of shells an atom contains. For example, the radius of the atoms of the elements within a particular group in the periodic table increases from the top of the group to the bottom. 

Although each of the elements described in Problems 2.24 and 2.25 has a unique electron configuration, each has the same outermost electron configuration. Since they are in the same group of the Periodic Table, and therefore have similar chemical properties, quantum mechanics has explained, most importantly, that the chemical properties of the elements depend not on the total number of electrons (or atomic number) but principally upon the configuration of the electrons in the outermost shell. This means the group number is in fact the number of electrons in the outer shell of the representative elements, and the principal quantum number is identified with the period. 

The second approach to be considered is a fiu better candidate for the claim to explain the periodic table from quantum mechanics. Even if the crude notion of a particular number of outer-shell electrons for any particular atom faik to give a fundamental explanation, it should be possible to carry out detailed calculations that allow atoms to have more complicated configurations. Going to such a deeper level than the notion of a particular number of electrons in sheik might thus provide a more successful explanation of the periodic system. 

Moreover, a reduction of chemistry, or more specifically the periodic table, to quantum mechanics requires far more than a mere approximate explanation of the properties of elements in terms of outer electron configurations. After all, quantum mechanics or atomic theory , which the authors constantly allude to, is not a qualitative theory dealing in outer-shell electrons. Such explanations are indeed frowned upon by physicists as being of a typically picturesque and naively realistic kind, typical of chemists. Worse still, according to quantum mechanics, the very notion of electron shells or electron configurations becomes strictly invalid as mentioned in the introduction. ... 

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