Discovery of Periodic Tables for Molecules
The periodic table for elements was proposed in the year 1869, which thereafter became one of the cornerstones of the chemistry & natural sciences. The periodic table was designed to contain all the elements or say atoms, found in nature in a special layout that groups the elements in rows and columns according to one of their most important characteristics, i.e, the number of electrons. Researchers have used the element periodic table for decades in order to predict the characteristics of the then-unknown elements. And these elements are added to the table over time.
Now, Could there be such a periodic table for molecules? Although some scientists have thought about this possibility of periodic table for molecules and proposed periodic rules for predicting the existence of some molecules, these predictions by the researchers were valid only for certain clusters of atoms with a quasi-spherical symmetry, which is because of the limitations of their own theory. However, there are many clusters of atoms with other shapes and other types of symmetries which has to be accounted for with a better model. Thus, a researchers team from Tokyo Tech, including Dr. Naoki Haruta, Dr. Takamasa Tsukamoto, Prof. Kimihisa Yamamoto, and their colleagues, proposed a new approach to build a periodic table for molecules considering multiple types of symmetries of molecules.
This approach by the team is based on a keen observation on the behavior of the valence electrons of atoms that form the molecular clusters. The valence electrons can be regarded as free electrons in atoms with an outermost orbital, and thus these can interact with the electrons of other atoms to produce compounds. And when multiple atoms form a cluster with a symmetrical shape, their valence electrons tend to occupy specific molecular orbitals which are called as super-atomic orbitals, where the cluster behaves almost exactly as if they were the electrons of huge atoms.
When this fact is considered and analyzed the effects of the structural symmetries for clusters, the scientists proposed symmetry-adapted orbital models, that are in agreement with multiple known molecules as well as state-of-the-art quantum-mechanical calculations. The new periodic tables, which would be created for each symmetry type, would actually be four-dimensional, because the molecules would be arranged according to four parameters: groups and periods (based on their valence electrons, similar to the normal periodic table), species (based on the constituting elements), and families (based on the number of atoms).
The symmetry-adapted orbital (SAO) approach is very promising in the field of materials design. Prof. Yamamoto stated that modern synthesis techniques enable us to produce many innovative materials based on the symmetry-adapted orbital model, such as lightweight magnetic materials. The road ahead for researchers lies in further expanding these periodic tables to molecular clusters with other shapes and symmetries and predicting stable molecules that have yet to be developed. Prof. Yamamoto concluded that among the infinite combinations of constitutive elements, the proposed periodic table for molecules will be a significant contribution to the discovery of novel functional materials.