Periodic Table For Molecules | Based On Symmetry-Adapted Orbital Models

  • Researchers propose a new methodology to develop a periodic table for molecules with different types of symmetries. 
  • This new approach is strongly supported by many quantum mechanical calculations and various existing molecules. 

In 1869, a Russian chemist developed the first recognizable periodic table to illustrate periodic trends of the then-identified elements. The idea was to arrange the elements by atomic number, electron configuration, and chemical properties.

The table has been gradually expanded and improved with the discovery of new elements and theoretical models that describe chemical attributes. Today, it is widely used as a useful framework for examining chemical reactions.

Could there be a similar table for molecules?

Some scientists have already thought about this and suggested certain principles to predict the existence of specific molecules. However, these principles have their own limitations and they could be applied only to groups of atoms with a quasi-spherical symmetry.

But since many groups of atoms have other types of symmetries and shapes, it is possible to build better models for such clusters of atoms. Keeping this in mind, researchers at Tokyo Institute of Technology have proposed a new methodology to develop a periodic table for molecules with different types of symmetries.

Attributes of New Periodic Table For Molecules

The approach of developing a new periodic table is based on how atoms’ valence electrons behave, which plays a key role in the formation of molecular clusters.

Valence electrons are the electrons located at the atom’s outermost shell. Since these electrons are the first ones to participate in the formation of a chemical bond, they determine chemical properties of an atom, such as whether it may bond with other atoms, and if so, how quickly and with how many.

When two or more atoms create a symmetrically-shaped cluster, their valence electrons usually occupy particular molecular orbitals known as super-atomic orbitals. In this case, these electrons act as if they were the electrons of a large atom.

The research team analyzed the symmetrical shapes of clusters and came up with ‘symmetry-adapted orbital models’. Instead of just assuming that all shapes of clusters are almost spherical, these models take into account the level splitting of the electronic orbitals due to lower structural symmetries.

Orbital patterns for various structural symmetries

In simple terms, the proposed model accounts for orbital patterns that obey specific rules for different types of symmetries, including icosahedral, octahedral and tetrahedral. Also, the new model is strongly supported by many quantum mechanical calculations and various existing molecules.

Reference: Nature Communications | DOI:10.1038/s41467-019-11649-0 | Tokyo Tech

Researchers proposed periodic tables based on each type of symmetry. The tables would be 4-dimensional in which molecules would be ordered according to 4 factors:

  1. Groups (based on valence electrons)
  2. Periods (based on valence electrons)
  3. Species (based on elements participating in the chemical reaction)
  4. Families (based on the number of atoms)

A periodic table for different types of tetrahedral molecules

The new models seem quite promising in terms of materials design. The proposed periodic table is practical for the systematic exploration of unknown stable clusters.

Read: Does Inverted Periodic Table (Upside Down) Make More Sense?

Among with countless combinations of constitutive atoms, it will help chemist form innovative materials based on clusters with optical, magnetic, and catalytic functions.

Written by
Varun Kumar

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