Helium placement in the Periodic Table For crystallography structure. By 2019, one hundred and fifty years after Dmitry Mendeleev published the first successful version of the Periodic Table of the chemical elements, there was still no universal agreement regarding what a chemical element is. A notable indication of the ongoing ambiguity is IUPAC’s Gold Book, which allows two different versions of the term ‘chemical element. (1) a species of atoms – all atoms with the same number of protons in the atomic nucleus; (2) a pure chemical substance composed of atoms with the same number of protons in the atomic nucleus. W. H. E. Schwarz in his 2007 paper (Schwarz, 2007) argued that there are in fact three different definitions of a chemical element usually encountered: (1) a basic chemical element; (2) a metallurgical element or simple material; (3) an astrophysical spectroscopic element or elemental atom. W. B. Jensen suggested a definition (Jensen, 1998) which focuses on atomic nuclei rather on
neutral atoms: (1) a class of nuclei, all of which have the same atomic number. The one question that permanently accompanies the definition of the chemical element is the representation of the Periodic Table itself.
The most common version of the revered icon of chemistry is the IUPAC Periodic Table of the Elements. Whichever representation of the periodic system is argued to be the optimal one (Leigh, 2009; Scerri, 2009), consistency of representation is the criterion that has to be met. The IUPAC Periodic Table has four blocks of chemical elements: the s-, p-, d– and fblocks, hence its whole body is based on electron configurations. One hundred and seventeen of the known elements fit into those blocks; however, there is only one element, helium, placed on top of the p-block as it is a noble gas. Hence, the representation becomes inconsistent overall because the Periodic Table simultaneously adopts two different definitions of the chemical element. According to the Gold Book, we then have one hundred and seventeen species of atoms and one pure chemical substance. Thus, such a placement of helium transforms the Periodic Table of Chemical Elements into the Periodic Table of Pure Substances.