Revolutionary attempts to solve the troubles of the periodic table

 Introduction

The periodic table of the elements is one of the most powerful icons in science ; a single document that consolidates much of our knowledge of chemistry.For over a century, millions of children have started out every year on their journey into the fascinating world of chemistry with the periodic table of the chemical elements. Hundreds of versions of the periodic table have been proposed, and a variety of methods have been applied to its analysis.Throughout its long history, the periodic table has been disputed, altered and improved as science has progressed and as new elements have been discovered. But despite the dramatic changes, no revolution in the basic nature of the periodic system. After evolving for over 200 years through the work of many people, the periodic table remains at the heart of the study of chemistry  It ranks as one of the most fruitful ideas in the modern science.

Efforts of many physicists and chemists 

The modifications to quantum theory made by Werner Heisenberg and Erwin Schrodinger in the mid - 1920s yielded quantum mechanisms.  Despite the efforts of many physicists and chemists  quantum mechanics cannot explain the periodic table any further. 

For example : 1) It cannot explain from first principles the order in which electrons fill the various electron shells. 

2) The electronic configurations of atoms  on which the modern understanding of the periodic table is based. 

Above two examples can't be derived using quantum mechanisms(this is because the fundamental equation of quantum mechanics the Schrodinger equation, cannot be solved exactly for atoms other than hydrogen). 

Relationship between periodic table and quantum mechanism

The periodic table has been around for almost 140 years and has survived many revolutionary discoveries including that of subatomic structure and the development of quantum mechanics and relativity. The modern explanation why the elements fall into vertical columns, showing similar properties, has been provided by quantum mechanics and describes the shell structure of electrons orbiting the nucleus of each kind of atom. Mendeleev 's first published table of 1869 initiated an era of chemical research that was both guided by  and capsulized in the evolving tabular forms for displaying chemical periodicities. Only with Bohr' s 1913-1923 introduction of the "old quantum theory" and the final discovery of Schrodinger 's wave mechanics in 1925 would the periodic table be supplanted as the deepest expression of current chemical understanding. From the modern viewpoint, quantum mechanics gives a clear rationale for the serial filling of atomic energy levels, according to known chemical periodicity patterns. Thus, one may conclude that the most profound characterization of the chemical properties of a given atom is in terms of quantum numbers or equivalent descriptions that allow the relative energy, angular shape  radial diffuseness or other properties of its occupied or unoccupied valence orbitals to be inferred.The left - step table, as first proposed by Charles Janet in 1929,has been the subject of much discussion. Since it is said to reflect the quantum mechanical understanding of the periodic table to a greater extent than the conventional medium - long form table.

Janet’s left-step table

 

A parallel discussion concerns the precise membership of group 3, an issue that raises many notions that lie at the heart of the modern periodic table such as the nature of electron configurations.

Need for change

There are a number of elements whose placement in the periodic table have been debated by generations of chemists. These elements include hydrogen, helium,lanthanum, actinium, lutetium and lawrencium. The ground state electronic configurations of the atoms of these elements do not resolve these questions contrary to the perceived dogma in chemistry that electronic configurations explain everything about the elements. The question of  where to place helium is one sign of trouble in the periodic table. Even the first element, hydrogen, has been causing trouble for some time. It can both lose as well as gain an electron. As a result it can be placed in group 1, or with halogens in group 17.

Long form of periodic table : showing positions of the hydrogen and the helium.

 Another form of trouble lies in group 3. Many textbook and wall-chart periodic tables show group 3 as consisting of the elements scandium ,yttrium ,lanthanum and actinium. A similar number of tables show a difference in the last two elements by featuring lutetium and lawrencium instead. Finally, the synthesis of super heavy elements over the past 60 years or so, and in particular the synthesis of elements wth atomic numbers beyond 103 has raised some new philosophical questions regarding the status of the periodic law. In these heavy elements relativistic effects contribute significantly to the extent that the periodic law may cease to hold. For example, chemical experiments on minute quantities of rutherfordium (104) and dubnium(105) indicate considerable differences in properties from those expected on the basis of the groups of the table in which they occur. However, similar chemical experiments with seaborgium(106) and bohrium(107) have shown that the periodic law becomes valid again in that these elements show the behavior that is expected on the basis of the periodic table.