Construction of the periodic table

In section 1.1 Atomic nucleus, electrons and orbitals we saw that an atom in its ground state consists of an equal amount of electrons and protons and that the electrons are located around the nucleus in different orbitals. These orbitals have different levels of energy which determine where the individual electrons will be hosted. In this section we are going to look at how the elements are arranged the periodic table and why the periodic table has its actual configuration. 

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The discovery of individual elements was necessary to begin to the construction of the periodic table. 

Concepts of valence and the construction of the periodic table have always been associated with one another. As the idea of electronic distribution in orbitals developed, the role of the noble gases and the concept of filled shells were stressed in many theoretical developments. It must be emphasized that most theoreticians never thought in terms of an absolute prohibition of reactions for the noble gases. Indeed, for awhile the possibilities of compoimd formation for xenon and even for krypton were suggested. It was only after a number of claims for the synthesis of compounds were discredited and the best investigations of such... 

The discovery of the individual elements was a necessary prerequisite for the construction of the periodic table. The first pure elements have been known since the time of the Ancient Greeks who used the metallic elements gold (Au), silver (Ag), tin (Sn), copper (Cu), lead (Pb), and mercury (Hg). The first individual credited with the discovery of an element was Hennig Brand, a German scientist who discovered the element phosphorous in 1649. There were 63 known elements in 1869, the year Mendeleev created the periodic table. 

Valence was recognized as a periodic property by Mendeleev, and was used in his construction of the periodic table in 1869 yet this development threw no new light on the origin or character of valence forces. For that, chemists had to await the elucidation of atomic structure. 

The orbitals with lowest energy level are filled first. The following examples sketch the electron configuration for all elements. That way the aufbau principle and construction of the periodic table will be clear. 

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 the early 1700s, the alchemists were unaware of the defining role that electrons played in bond formation and transformation of chemical species. The structure of the nucleus, elucidation of bond formation with electron sharing and the construction of the periodic table were major triumphs of the chemical and physical sciences in the nineteenth and early twentieth centuries [6, pp.113-166]. Before these developments, chemical transformation of the elements themselves, such as lead to gold, was on the table as an early goal of research. 

Interestingly, as a so-called Dobereiner triad (i.e., a triad of elements, whose chemical similarities were recognized by German chemist Johann Wolfgang Dobereiner in the nineteenth century), sulfur, selenium and tellurium played a role in the initial construction of the periodic table. 

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