From the Archives
The Proper Place for Hydrogen in the Periodic Table
by Marshall W. Cronyn ’40
A puzzling mystery attends this title. Quite remarkable, after more than 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…R.T. Sanderson has pointed out the essential elements of this puzzle:
Electronically, or by valence, hydrogen belongs in both groups I and VII. Its electronegativity, however, corresponds to a chemistry intermediate between the two extremes. Hydrogen is, therefore, placed above the other elements and just to the left of carbon. In recognition of its fundamental electronic dissimilarity to both the alkali metals and the halogens, and of its similarity in electronegativity to the elements of group M4, especially carbon, hydrogen is placed over that group but in a separate independent position. Thus hydrogen, whose outer shell is half filled, is close to carbon whose outer shell is half filled. These two, alone of all the elements, have neither outer valency nor outer electrons left over when all possible covalent bonds have been formed.
[Marsh cites several unhappy attempts to place hydrogen.]
It has been said that “the periodic table is also deeply reassuring in that it accounts for and assigns a specific position to every element.” However, contrary to that sanguine sentiment, the search for an appropriate chemical family for hydrogen seems to have been abandoned altogether. . . .
The typical periodic table in textbooks and in the usual chemistry classrooms charts places hydrogen above the alkali metals. Sometimes it carries a slightly different color, perhaps a blush of embarrassment for such an awkward and inappropriate arrangement. Its chemistry is quite obviously not metallic and is treated quite separately from that of the alkali metals.
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. How could this be so? For in the beginning, of both time and the periodic table, there was hydrogen. Still the most abundant element in the entire universe, about 88.67% of all atoms, its fusion brightening our days from time to time, while its oxidation product fills in the rest, and not just April. It is the third most abundant element of the earth’s crust, about 15.47% of all crustal atoms. This little element with only one electron can claim, above all other elements, to be a constituent of the largest number, by a minuscule margin, of all compounds both natural and synthetic.
Numerology Versus Chemistry
Why has such a wealth of chemistry been ignored repeatedly in discussions of hydrogen’s place in the periodic table? It would appear that the most persistent error in the assignment of hydrogen to its place is a kind of numerology, the belief that simply the number of electrons, absent context, determines the chemistry of an element. The fallacy of this belief is most readily exposed in a classic non sequitur: helium, because it has two outer shell electrons, should therefore be placed above beryllium in the periodic table. The assignment of hydrogen to the alkali-metal family of elements, because it has one electron in its outer shell, is no less absurd than it would be to place helium over beryllium because it has two outer electrons. Both statements omit context for these electrons and belong to numerology and not chemistry. As Sanderson pointed out, the context for hydrogen is that of a half-filled shell while for helium it is, of course, that of the filled valence shell which places it as the first member of the noble gases. [Marsh contrasts the ionization potential of hydrogen with other metals.] Hydrogen is clearly no more metallic than mashed potatoes.