2. Mendeleev Interpreted by Schrödinger and Lewis
Select "electron configuration" to show the following information:[more]
On the right is displayed a qualitative model of the predicted electronic configuration, obtained following the Aufbau principle, the Pauli exclusion principle and Hund's rule. According to Hund's rule, electrons occupy orbitals starting from the empty orbitals before doubly occupying them, and the most stable configuration results when the spins are parallel.
The correct energy level sequence can be highlighted with the "energy level" checkbox. The shift depending on the atomic number can be highlighted with the "energy shift" checkbox.
The yellow zone on the upper right of the models indicates that there are more orbitals with higher energy that have been omitted. For each , there is clear separation between the valence shell, the outermost occupied shell and the core shell, in which the electrons have a noble gas–closed shell structure, with pairs indicated by an .
On the bottom left is the valence shell data from the Wolfram database. Some discrepancy can be found between the predicted theoretical model and the actual data; for example, for Cr (). This is due to the enhanced stability with a larger number of parallel spins (Hund's rule) in the actual configuration. Other discrepancies can be related to more complex effects, including spin-orbital interactions.
The top-left part illustrates the principles underlying the construction of the periodic table. Each row corresponds to an energy level: when it is fully occupied, the labels turn from magenta to black. When a noble gas–like configuration is achieved, the next row starts filling. Each column must be related to a particular orbital.
The filling follows the previous model for the electron configurations; however, the block is irregular. As a consequence, the block is split into two different blocks ( and ). When the block is filling, the number shown in magenta is the actual position in the block. When this block is fully filled, the split is maintained as the total number of electrons must be equal to 10 ().
The irregularity of the block is highlighted in red; the first element of this block (Ce, ) would be expected to have two electrons in the level, therefore the position is shifted by one (i.e., Yb with occupies the position of the block) . The position is occupied by the Lu with a configuration. La and Lu both have the last electron in , but Lu has a fully filled . Thus is highlighted in green.
Mendeleev's genius is highlighted by his correct placement in the atomic number sequence (despite the fact that he was aware only of the atomic masses). More spectacular was his prediction of yet-undiscovered elements, such as germanium .
Selecting "Lewis structures" shows the Lewis structures for the first 10 elements. The count of the unshared electrons, the empty orbitals and the lone pair is indicated. The "empty orbitals" checkbox helps visualize the empty orbitals with a dashed line. Be, B and C form chemical bonds by utilizing excited orbitals (indicated in green) by uncoupling a pair. In special cases (e.g. nitrates), oxygen can use an excited orbital by coupling a pair. In this case, both structures are shown.[less]
Snapshot 1: electron configuration of germanium (); its existence was predicted by Mendeleev
Snapshot 2: Electron configuration of oxygen (); two Lewis structures are shown: the ground state and an excited state. On the periodic table, the first subshell is completed.
Snapshot 3: electron configuration of carbon (); two Lewis structures are shown: the ground state and an excited valence state, which is used to form chemical bonds
 P. W. Atkins and R. S. Friedman, Molecular Quantum Mechanics, 3rd ed., New York: Oxford University Press, 1997.
 H. Orofino, S. P. Machado and R. B. Faria, "The Use of Rich and Suter Diagrams to Explain the Electron Configurations of Transition Elements," Química Nova, 36(6), 2013 pp. 894–896. doi:10.1590/S0100-40422013000600027.
 E. R. Scerri, A Tale of Seven Elements, Oxford: Oxford University Press, 2013.