TRIATOMIC OR MONATOMIC HYDROGEN BY G . R . SCHULTZE
Articles dealing with the existence of triatomic hydrogen appear over and over again. Indeed the subject matter merits the greatest attention not only because of its chemical interest but also because of its relation to the problem of chemical valence. Further interest in this field has been occasioned by the recent developments in the field of wave mechanics, developments which have led to a very remarkable extension of our conceptions of interatomic forces and chemical bonds, particularly for homopolar compounds. The more one becomes acquainted with these problems the more one realizes how little has actually been achieved toward the establishing of conclusive proof of the existence of triatomic hydrogen. This note is written with special reference to a recent paper by J. L. Binder, E. A. Tilby and A. C. Grubb.l Without doubt the paper represents a good piece of research as far as a confirmation of facts already known and an extension of our knowledge of active hydrogen in general are concerned. But a proof of the existence of some sort of active hydrogen does not justify the assumption of the presence of triatomic hydrogen, since an explanation on the basis of monatomic hydrogen will still remain possible. R e know that the existence of monatomic hydrogen is well established by the fundamental investigations of Langmuir, Wood, and many others. The facts about monatomic hydrogen are summarized in an excellent monograph by Bonhoeffer.* On the other hand Paneth, Klever, and Peters3 have shown that the experiments presented by several investigators intended to prove the formation of triatomic hydrogen are open to question. They conclude that there really has been no proof to date for the existence of a neutral H3-molecule. Moreover the interpretation of the results obtained by Wendt and his co-workers4 is mainly based upon two facts: (a) the contraction of the gas under influence of activating agents; (b) the existence of H3+ as was found by J. J. Thompson.5 The contraction finds a satisfactory explanation in the ‘clean-up effect’. The positive ray spectrum undoubtedly gives evidence of a charged Ha+ion, but its existence is far from being a proof of an uncharged Ht molecule as Paneth (loc. cit.) has already pointed out. Another argument in favor of the triatomic modification is to be found in the claims of differences in reactivity and stability between H and H3. A ’Canadian Journal of Research, 4, 330 (1931). Cf. also Chem. Abs., 25, 3917 (1931). Erg. Exact. Naturwiss., 6,201 (1927). Z.Elektrochemie, 33, 102 (1927). J. Am. Chem. SOC.,42,930 (1920);44,510 (1922). Proc. Roy. SOC.,89A, I (1913);also cf. .4.I. Dempster: Phys. Rev., ( 2 ) 8, 6j1 (1916); Phil. Mag., 31, 438 (1916).
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higher or lower concentration of atoms and, in particular, the fundamental influence of wall poisoning upon the formation of active hydrogen readily explain any observed effects which all together would only represent differences in degree and not in kind. Thus the writer of this communication was able to estimate for the active hydrogen an increase of average life of about 5 to I O times as much as was assumed before simply by reducing the wall skin recombination by means of a negative catalyst.6 On this ground no definite assertion can be made for the existence of either H or HI unlesssome specific reaction’ for either one of the modifications is available. Additional evidence for the monatomic structure of active hydrogen could be drawn from more definite conceptions taken from the kinetic theory and from our knowledge of the reaction kinetics of disintegration of active hydrogen. However, we need not go too much into detail. One more point should be mentioned however. While the older theory of atomic structure permitted such a molecule as Ha8,the more recent developments of wave mechanics point to a contrary conclusion. The different forms of action of homopolar valence forces have been shown to be dependent upon the symmetry-properties of Schroedinger’s Eigenfunction. The actual number of valences computed from the multiplicity of terms which correspond to a certain state, is given by the group-theoretical possibilities as limited by Pauli’s Principle. Assuming certain resonance effects London has expanded his theory to the formation of hydrogen molec u l e ~and ~ has shown that only two possibilities are given: homopolar binding (Le. formation of Hz-molecules) or elastic reflexion (Le. H H atoms). For a third hydrogen atomlo an immediate application of Pauli’s Principle results in an exclusion of a state in which all three electrons are characterized in exactly the same way. Only if the two H-atoms were bound antisymmetrically could the third one be attracted. An antisymmetrical combination] however. corresponds in the last analysis to an exclusively positive value of energy as derived from Broglie’s equation which means repulsion of the atoms. In conclusion may I add that I do not want to deny the possibility of an active form of hydrogen which may be attributable to an H s molecule although modern physics has furnished strong theoretical arguments against such a molecule, but I do not believe that a mere proof of the existence of some sort of activation is sufficient basis for the assumption of the existence of H, when one has a possible explanation by means of the well established atomic modification of active hydrogen.
+
v. Wartenberg and G. Schultze: Z. physik. Chem., 6,261 (1930). Using perhaps Wrede’s orifice method if the two forms of active hydrogen should happen to be chemically identical. Cf. also Harteck: Z. physik. Chem., 139A, 106 (1928). a K. Bohr: Meddel. Vetenskapsakad. Kobelinst., 5 , No. 28, I (1919). W. Heitler and F. London: Z. Physik, 44, 4 j j (1927). F. London: Physik. Z., 29, 559 (1928); Science Abstr.: 31, 909 (1928) No. 3170; W. Heitler and F. London: Z. Physik, 44, 468 (1927). F. London: Z. Elektrochemie, 35, 5 5 2 (1929).
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Since this communication was sent in, the author’s attention has been called to an article by Egon Hiedemann (2. physik. Chem., 153A,2 1 0 - 2 4 0 (193 I)) entitled, “Die Natur des ‘dreiatomigen Wasserstoffes.” The elaborate experimental work of this investigator has shown conclusively that triatomic hydrogen does not exist. To summarize the result of his work: he found that a hydride of silicon accounts for the activity which can not be explained by means of monatomic hydrogen. I t would seem therefore that the question of the non-existence of triatomic hydrogen has been definitely settled. School of Chemistry, Unavmsity of Minnesota.
