Electron‐counting rules, three‐dimensional aromaticity, and the boundaries of the Periodic Table

To describe aromaticity of planar and three‐dimensional molecules, different electron counting rules are employed. Here, the relationship between the Hückel 4 n  + 2 rule and the Hirsch 2( n  + 1) 2 rule is established based on formal approach considering the electrons as objects in an arbitrary n ‐dimensional space of states . Two types of three‐dimensional aromaticity, referred to as ‘spherical’ (following the 4 n  + 2 electron counting rule) and ‘spatial’ (following the 2( n  + 1) 2 electron counting rule) are distinguished. A conclusion concerning the boundaries of the Periodic Table of Elements is made based on the same formal approach that no g ‐ (or higher) elements can exist and that possible extension of the Periodic Table beyond the seventh period must be followed by filling of the 8p or inner 6f or 7d levels. Copyright © 2011 John Wiley & Sons, Ltd.