26 No. 3
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The Placement of Hydrogen in the Periodic Table
by Eric Scerri
a recent article (Nov-Dec
2003 CI, p. 14), Peter Atkins and Herb Kaesz
proposed a modification to the periodic table concerning the
placement of the element hydrogen. Contrary to its usual placement
at the top of the alkali metals, and its occasional placement
among the halogens, Atkins and Kaesz choose to position hydrogen
on its own and floating above the table. In doing so these
authors appear to overlook the possibility of hydrogen’s
membership of the group that is usually headed by carbon as
has recently been argued in detail.1
|The proposed format of the periodic table, with hydrogen at its head in Period 1 but assigned to no group.
But rather than considering the relative virtues of these three possible placements, I intend to consider the argument for the removal of hydrogen from the main body of the table a little more closely and from the perspective of the philosophy of chemistry. A very widely held belief, among chemists and others alike, is that the periodic system consists primarily of a classification of the elements as simple substances that can be isolated and whose properties can be examined experimentally. However, there is a long-standing tradition of also regarding the elements as unobservable bearers of properties, sometimes termed elements as basic substances.
Surprising as it may seem, this notion has had a profound influence on the establishment and the survival of the periodic system since its discovery in the 1860s. For example, Mendeleev, arguably the leading discoverer of the periodic system, frequently stressed that he was primarily classifying the elements in the sense of abstract bearers of properties and not as simple substances. Whereas carbon occurs as graphite and diamond, if one focuses on the element in its simple substance forms, the entry for carbon in the periodic table refers to the element as the basic substance which underlies both allotropic forms.
Similarly, the discovery of isotopes in the early years of the twentieth century led to a crisis in which the known “atoms” appeared to have suddenly multiplied in number. Some chemists like Kasimir Fajans stated that the periodic table would not survive this discovery. However, the radio-chemist Fritz Paneth argued, like Mendeleev, that the periodic table should not primarily classify simple substances like the isotopes of the elements. He argued that it should classify the abstract elements and that the periodic table would survive the discovery of isotopes, which of course it has.
Our current inability to place hydrogen in the periodic table in an unambiguous manner should not lead us to exclude it from the periodic law altogether, as Atkins and Kaesz seem to imply in removing hydrogen from the main body of the table. I suggest that hydrogen is as subject to the periodic law as all the other elements are. I also maintain that there is a “fact of the matter” as to the optimum placement of hydrogen in the main body of the table and that this is not a matter of utility or convention that can be legislated as these and other authors have argued.
than relying on specific properties of the elements as simple
substances, we should seek some form of underlying regularity
in order to settle the question of the placement of hydrogen.
One possibility is to begin each new period with a new value
of n + l (sum of first two quantum numbers) in the
assignment of electronic configurations, as many authors have
suggested.3 Such a periodic table
results in the placement of hydrogen in the alkali metals
and helium among the alkaline earths. Of course the second
of these placements raises further difficulties for those
who insist on viewing the elements only as simple substances.
But as I have tried to argue here this is a mistaken view
of the nature of the chemical elements.
1. M. W. Cronyn, J. Chem. Educ., 2003, 80, 947–951.
2. E. R. Scerri, in Minds and Molecules, N. Bhushan, S. Rosenfeld (eds.), Oxford University Press, New York, 2000.
3. G. Katz, The Chemical Educator, 2001, 6, 324–332.
Eric Scerri <[email protected]>, Department of Chemistry and Biochemistry, University of California in Los Angeles, CA, USA.
I support your suggestion for the new central position for hydrogen. It is a good and reasonable solution to the problems you discuss.
—Brad Bovenzi <[email protected]>, McQuaid High School, Rochester, New York, USA
To take away H from the head of the alkaline metals group and put it in the center is an excellent idea and should be adopted by IUPAC.
—Alex von Zelewsky <[email protected]>, Department of Chemistry, University of Fribourg, Perolles, Fribourg, Switzerland
I do agree with you that an adaptation in which hydrogen is centered at the head of the periodic table has great merit. You have sound arguments for such a proposition.
—Primož Šegedin <[email protected]>, Department of Chemistry and Chemical Technology, University of Ljubljana, Slovenia.
last modified 17 May 2004.
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