Quite remarkable, after 130 years of construction, the place of hydrogen in the periodic table is still the subject of doubt, confusion, and inadequate explanation that appears to be little more than numerology.
Despite Cronyn’s somewhat hyperbolic language, it is indeed true that the position of hydrogen remains a contentious issue. So much so, almost this whole chapter is devoted to the problem.
Hydrogen Location: An Overview
First, it should be noted that the hydrogen location problem is partially an artifact from using the rectangular short form or long form of the Periodic Table. Other designs, such as spiral or circular tables, enable the “H” symbol to be placed at the center, making the issue is moot. One such hydrogen-centered Periodic Table was proposed by Piutti in 1925 [3] while a more elegant spiral design was drawn by Griff in 1964 [4] (Figure 3.1).
Figure 3.1 Griff’s spiral Periodic Table of 1964 with “H” at the center (from Ref. [4]).
However, as the conventional Periodic Tables are almost ubiquitous, placement in these “standard” tables will be the focus of this chapter. While other elements are largely locked into place by the ordering of the atomic number, the fact that hydrogen and helium are above the next eight-member Period results in this dilemma.
Before discussing the pros and cons of each option, it is useful to the Reader to see the common alternatives listed. Most of these options have been reviewed by Petruševski and Cvetković [5]:
•Hydrogen as a member of Group 1
•Hydrogen as a member of Group 17
•Hydrogen as a member of Group 1 and Group 17
•Hydrogen as a member of Group 13 or 14
•Hydrogen as a member of Group 1 and Group 14 and Group 17
•Hydrogen on its own
•Hydrogen as a member of Group 1 and helium as a member of Group 2
Hydrogen as a Member of Group 1
Most commercial Periodic Tables show hydrogen as a member of Group 1. Cronyn has described the situation elegantly [2]:
So there is poor hydrogen, denied a chemical family to call its own, thrust like an unwanted orphan into a foster home where its chemistry cannot even be discussed in the same breath with the alkali metals where it now resides.
The strong argument for the placement of hydrogen in Group 1 is that of electron configuration, as championed by Scerri [6]. As the alkali metals have a valence electron configuration of ns1, then, as the ground state electron configuration of the hydrogen atom is 1s1, it follows that hydrogen should be placed at the head of the Group 1 column (Figure 3.2).
There are some supporting chemical arguments. In theory, the hydrogen atom ionizes to give a (positive) hydrogen ion, just as an alkali metal atom ionizes to give an alkali metal cation. Of course, hydrogen does not, in a chemical context, produce a free proton but is, instead, part of a positively charged network: [H(OH2)n]+. There is the similarity in that, in aqueous solution, the alkali metal ions, too, are all strongly aquated.
Figure 3.2 A traditional long form of the Periodic Table with hydrogen in Group 1 (from Ref. [6]).
Nevertheless, the alkali metal group form one of the best examples of systematic group trends. To place hydrogen at the top of it implies that hydrogen, itself, is an alkali metal. Yet every comprehensive inorganic book discusses the chemistry of hydrogen in a whole separate unit from that of the alkali metals [7].
Claims of the metallic nature of hydrogen under exceptionally high pressure are used to support the argument that hydrogen is actually an anomalous alkali metal. It was in 1935 that Wigner and Huntingdon were the first to propose on theoretical arguments that under extremes of compression, dihydrogen would undergo a transition to a metallic allotrope [8]. Much high-pressure research has been attempted in order to obtain physical evidence of the existence of metallic hydrogen. In part, such a search relates to the possibility of metallic hydrogen being in the core of the gas giant planets, Jupiter, Saturn, Neptune, and Uranus.
However, the goal of some researchers has been to provide evidence that hydrogen really should be considered as an alkali metal [9]. There has been a claim that solid, metallic hydrogen has indeed been produced [10]. However, does the formation of a metallic allotrope under such extreme conditions really provide evidence that hydrogen belongs to the alkali metal family? Moore has pointed out the fallacy of that argument [11]:
Boron, oxygen, sulfur, selenium, tellurium, and phosphorus all can be made conductive under pressure, but only in the case of hydrogen is metallization thought to vindicate its predicted properties.
The most intriguing evidence for hydrogen as a member of Group 1 has come from a different route. A compound has been synthesized containing a four-coordinate hydrogen atom that occupies the same site as lithium or sodium [12].
Hydrogen as a Member of Group 17
It can be argued that the 1s1 of hydrogen is one electron short of a complete electron shell, just as are the ns2np5 electron configurations of the halogens. However, the reasons for hydrogen placement as a member of Group 17 rely more on the chemical arguments. Supporting evidence for placement in Group 17 is that hydrogen, like the halogens, forms a stable diatomic molecule. By contrast with the halogens, dihydrogen is not as reactive as the dihalogens.
Also similarly, hydrogen can form a negative, hydride ion. Sacks argued vociferously for the placement of hydrogen unambiguously in Group 17 [13]:
A Coulombic model, in which all compounds of hydrogen are treated as hydrides, places hydrogen exclusively as the first member of the halogen family and forms the basis for reconsideration of fundamental concepts in bonding and structures.
But again, similarity in formula masks a completely different chemical behavior. In particular, the hydride ion decomposes violently in the presence of water, unlike the water stability (albeit with hydrolysis in the fluoride case) of the halide ions.
Hydrogen as a Member of Group 1 and Group 17
At this point, the discussion will be
