Kossel, Walther (Ludwig Julius Paschen Heinrich)

(b. Berlin, Germany, 4 January 1888; d Kassel, Germany, 22 May 1956)

physics.

Kossel was descended from an old family of distinguished scholars. His father, Albrecht Kossel, longtime professor of physiology at the University of Heidelberg, received the Nobel Prize in physiology or medicine ion 1910 for “contributions to the chemistry of the cell throuogh his work on proteins, including the nucleic substances.” His mother’s maiden name was Holtzmann. The atmosphere in the parental home fostered in young Kossel two characteristics that always appeared in his work—his delight in a careful, well-ordered style and his thorough clarity of presentation. He considered these traits crucial to a scientist, and he took for his life’s work an interest in “that which most intrinsically holds physics together.”

After attending the Gymnasiums at Marburg and Heidelberg, Kossel studied physics under Philipp Lenard at the University of Heidelberg. There he was an assistant in physics from 1910. He received his doctorate in 1911 with a dissertation on an experimental investigation of the character and quantity of secondary cathode rays produced in different gases by primaries of diverse velocities (1). In the same year, he married Hedwig Kellner.

In order to advance his knowledge of physics he then moved to Munich, where Roentgen and Sommer- feld at the University, and Zenneck at the Technische Hochschule, presided over othriving institutes where P. S. Epstein, P. P. Ewald, W. Friedrich, M. von Laue, P. Knipping, P. P. Koch, and E. Wagner were working. In the spring of 1912, Laue, Friedrich and Knipping had shown that X rays could be diffracted by crystals, thereby opening new avenues ion the physics of X rays and providing a new method for exploring the structure of crystals. Soon after, Sommerfeld, under whom Kossel had worked, began his explorations of atomic structure and spectral lines within the framework of Bohr’s quantum theory of the atom. Besides remaining in touch with these developments, Kossel, who became Zenneck’s assistant in 1913, mastered the then emerging field of electronics, which enabled him to develop radio amplifying tubes for use in World War I.

Kossel’s first important contribution to physics was his extension of Bohr’s theory to the mechanism of X-ray emission (2). According to Kossel, who here succeeded where both Bohr and Moseley had failed, characteristic high-frequency (X) radiation accompanies the binding of an electron into a prior vacancy within the atom. The deeper the hole and the greater the distance through which the electron falls the higher the frequency of the emitted quantum. This picture, as developed especially by Sommerfeld, brought a general understanding of the X-ray spectrum, an estimate of the number of electrons n_j supposed arranged in concentric rings about the nucleus, and the recognition (in contrast to Bohr’s original system) that the normal atom contains electrons characterized by more than one quantum of angular momentum h/2π. But Kossel’s theory also had its difficulties, especially conflicts between (a) the calculated n_j and their values as inferred from chemical evidence and (b ) computed and observed frequencies of X-ray lines. Kossel later recognized (3) that (a ) arose from the working of a selection principle which Coster and Wentzel then fully specified; as for (b), Kossel left it to others, and contented himself with establishing n, on chemical grounds.

Kossel discussed the n_j in the context of the theories of valence and bonding, to which he made central contributions (4), Physicists had already tried to relate the electronic structure of atoms to two fundamental chemical phenomena—the chemical bond, that is, the attraction between atoms in a molecule, and valence, the quality that determines the number of atoms or groups with which any single atom or group will unite chemically, and also expresses this combining capacity relative to the hydrogen atom. In particular, J. J. Thomson, in his well-known Corpuscular Theory of Matter (1907), had given a theory of heteropolar bonding based upon the transfer of electrons from one molecule partner to another. The subsequent work of Rutherford, Bohr, and Moseley, which established the doctrine of atomic number, made possible more precise correspondences than Thomson had been able to suggest, and inspired Kossel to update the theory.

According to Kossel, who here followed Bohr and not Thomson, the number of electrons in the outer ring, the so-called valence electrons, determines the chemical properties of an atom. Kossel postulated that the extraordinary stability of the noble gases (then called inert gases) was due to the “closed” or complete nature of their outer ring or shell; there are eight electrons in the outer shell of all these gases except for helium, which has two. He believed, with Thomson, that metals achieve the stable electronic configuration of the nearest noble gas by losing electrons, and nonmetals achieve it by gaining electrons. The electrons lost by the metal are transferred to the nonmetal, and the resulting ions—cation (or positive ion) in metals and anion (or negative ion) in nonmetals—are held together by electrostatic (Coulombic) attraction. The theory has since been completely confirmed.

A month after the publication of Kossel’s paper (1916), the American chemist G. N. Lewis, working independently of him, published a paper (5) dealing with electrovalent compounds and especially covalent compounds, that is, those formed by the sharing of elections. The ideas of Kossel and Lewis did not achieve the immediate success among chemists that they deserved, possibly because of the interference of World War I with scientific activity. Their ideas achieved general acceptance in 1919 largely because of Langmuir’s systematizing efforts. Since then the theory has been developed and extended by many scientists, in particular, Sidgwick (6) and Linus Pauling (7). Kossel also helped extend it in an impor. tant paper written in collaboration with Sommerfeld (8). They showed that the spark spectrum of a given element (that is, the spectrum of its positive ion) has the same structure as the arc spectrum of the element one below it; apparently the electronic superstructure of the ion of the first element is identical in form to that of the atom of the second.

In 1920 Kossel became privatdocent at the Technische Hochschule, Munich. The following year he was called to the University of Kiel, where an excellent tradition in physics already existed, both Lenard and Dieterici having held chairs there, At Kiel he became professor ordinarius of theoretical physics and director of the Institute for Theoretical Physics. In addition to his scientific activities, Kossel offered his services to the academic administration of the university; in 1926 he served as dean of the Faculty of Mathematics, and in 1929-1930 he was appointed rector of the university. He continued his interest in administrative affairs throughout his life and later represented the University of Tubingen in the union of universities (Hochschulverband).

At Kiel, Kossel continued his work on valency and published a second edition of his book Valenz. kräfte und Röntgenspektren (9), He further developed the idea that the chemical bond was due for the most part to electrostatic forces, and he applied this idea to investigating the growth of ionic crystals. He turned increasingly to X-ray exploration of crystals and their growth, a subject which appealed to him esthetically. In 1927 he completed his first work on crystal growth (10), in which he dealt only with simple structures but in a way that furnished the basis for much recent work.

In 1932 Kossel went to Danzig (now Gdańsk, Poland), where he became professor ordinarius of experimental physics and director of the Institute for Experimental Physics at the Technische Hochschule. He made the Institute a place of lively, productive activity. In Danzig he found better opportunities to explore new fields, and he soon acquired a large school of students and co-workers, with whom he maintained friendly contacts throughout his life. Among his most important work at this time was his discovery of the interference effects (the so-called Kossel effect, first announced in 1935) produced by characteristic Roentgen rays excited in a single crystal. He also demonstrated the interference of electrons in con. verging beams, which gave great impetus to the theory of electron diffraction. For these and previous work Kossel was awarded in 1944 the Deutsche Physika. lische Gesellschaft’s highest honor—the Max Planck Medal. In 1955 he was made an honorary member of the Society. Additional honors followed including offers, which he declined, from the Universities of Berlin (1939) and Strasbourg (1942).

Kossel’s activity at Danzig was halted by the Russian occupation in 1945. He left with his family and most of the Physics Institute. Kossel moved the Institute’s costly equipment to the West. After a troubled time he found a new base of operations for his research in 1947, when he was appointed professor ordinarius of experimental physics and director of the Experimental Physics Institute at the University of Tübingen.

At Tübingen, Kossel and his colleagues worked in electron and optical diffraction, electrical discharge in gases, solid-state physics, acoustics, and crystal structure. He used large spherical single crystals of metal, of the kind first produced by his old Heidelberg friend Wilhelm Hausser. In the gas discharge he discovered continuous Lichtenberg figures, a phenom. enon of gas discharge physics unique in color and symmetry. In acoustics he continued the classical work of Helmholtz with the help of electronic methods. In collaboration with medical colleagues, Kossel developed a technique for measuring Roentgen dosage within the body. With his students he devel. oped electrostatic band generators with field voltages of up to 1.5 million volts and small disk generators with potentials up to 100,000 volts. In addition, Kossel continued to expend much time and effort on his experimental lectures.

Kossel was already suffering from the prolonged liver ailment which eventually claimed his life when he was grieved by his wife’s death in 1953. But he was mentally clear and active until the very end and as late as 1955 he lectured by special invitation in Paris. His last work, Individuation in der unbelebten Welt (11), published during his final illness, gives some idea of the range of his scientific activities.

Kossel was a corresponding member of the Göttingen and Halle Academies, an honorary member of the Deutsche Chemische Gesellschaft, the Deutsche Mineralogische Gesellschaft, the Verband Deutscher Physikalischer Gesellschaften, and the Bremen Natur. wissenschaftliche Gesellschaft. He was also an honorary citizen of the Christian-Albrechts University of Kiel.

BIBLIOGRAPHY

The works referred to in the text are

(1) “Über die sekundäre Kathodenstrahlung in der Nähe des Optimums der Primärgeschwindigkeit,” in Annalen der Physik, 37 (1912), 393-424.

(2) “Bemerkung zur Absorption homogener Röntgen. strahlen,” in Verhandlungen der deutschen physikalischen Gesellschaft, 16 (1914), 898 909.

(3) “Zum Bau der Röntgenspektren,” in Zeitschrift für Pliysik, 1 (1920), 119-134.

(4) “Über Molekülbindung als Frage des Atombaus,”. in Annalen der physik, 49 (1916), 229-362 (Received Dec., 1915). Partial English translations appear in II. M. Lei. cester, Source Book in Chemistry, 1900-1950 (Cambridge, Mass., 1968), pp. 94-100, and W. G. Palmer, A History of the Concept of Valency to 1930 (Cambridge, 1965), pp. 129-132.

(5) G. N. Lewis, “The Atom and the Molecule,” in Journal of the American Chemical Society, 38 (1916), 762-785 (Received Jan., 1916). Further details and develop. ments can be found in Lewis’ book Valence and the Struc. ture of Atoms and Molecules (New York, 1923; paperbound rep. ed., 1966).

(6) N. V. Sidgwick, The Electronic Theory of Valency (London. 1929).

(7) L. Pauling, The Nature of the Chemical Bond, 3rd ed.(Ithaca, N.Y., I960). Additional information can be found in G. V. Bykov, “Historical Sketch of the Electron Theories of Organic Chemistry,” in Chymia, 10 (1965), 199-253.

(8) With A. Sommerfeld, “Auswahlprinzip und Ver. schiebungssatz bei den Serienspektren,” in Verhandlungen der deutschen physikalischen Gesellschaft, 21 (1919), 240

(9) Valenzkräfte und Röntgenspektren (Berlin, 1924).

(10) “Zur Theorie des kristallwachstoms,” in Nach. richten der Akademie der Wissenschaften zu Göttingen, Mathematisch-physikalische Klasse (1927), 135 143.

(11) Individuation in der unbeleblen Welt (Berlin, 1956).

Most of the biographical information in this article is taken from two obituaries: “Zum Tode von Walther Kossel,” in Tübinger Chronik (26 May 1956), and E. N. da C. Andrade. “Prof. Walther Kossel,” in Nature, 178 (1956), 568-569. Details about Kossel’s fundamental work on X-ray spectra may be found in papers by J. L. Heilbron, from which the present account has been taken: “The Kossel-Sommerfeld Theory and the Ring Atom,” in Isis, 58 (Winter, 1967), 450-485; “The Work of H. G. J. Mose. ley,” in Isis, 57 (Fall, 1966), 336-364.

George B. Kauffman