The two limitations discussed in the section “Why a new periodic table” actually share a common origin: the number of lone pairs. These may appear either in the electronic configuration of isolated atoms (the exceptions) or in the valence state of atoms within molecules. Most proposed periodic tables—with only a few exceptions—are two-dimensional: the horizontal axis represents the classification criterion (valence or electronic configuration), while the vertical axis corresponds to the principal quantum number n.

We propose the addition of a third axis: the number of lone pairs, denoted D. From a chemical perspective, I have chosen to emphasize the valence v of an atom, defined as the number of unpaired electrons in its electronic configuration, as the horizontal key. The three-dimensional periodic table we introduce is therefore constructed on the Cartesian coordinate system (D, v, n).