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6 x 9 in. ; xiv 429 pp. ; 60 The Nature of the Chemical Bond. By LINWSPAWLING. figures. Ithaca, New York: Cornel1 University Press, 1939. Price: $4.50. This is a clearly written survey of the nature of chemical bonding as seen from the viewpoint of the atomic orbital method. I t is in this method that the wave mechanics connects most easily and naturally with the traditional ideas of chemists on the nature of chemical bonds and with the Lewis theory. Hence the presentation will have wide appeal and usefulness among chemists. The book is mainly a review of the author’s work on chemical bonding, with special emphasis on the study of bond distances in molecules and crystals, and on the use of the resonance concept in explaining bond structure, bond distances, and bond energies. Although based on quantum-mechanical ideas, the treatment is remarkably non-mathematical. After an introductory chapter on “Resonance and the Chemical Bond” comes a chapter on ionic character and electronegativity, based largely on a study of bond energies. This subject is peculiarly difficult to treat quantitatively, but the author has made great progress by his pioneer work and stimulating new ideas, which are further usefully developed in the book. Nevertheless, in the reviewer’s opinion, much probably remains t o be done before the subject will be in a wholly satisfactory state. Some specific criticisms may here be in order. The reviewer has shown theoretically (cf. equation 41 of a paper’ quoted by the author on page 66) that if one adopts with the author the postulate of the additivity of covalent bonds, then the amount of ionic character Q in a partially polar bond varies according to an equation of the type:
Q
)(zA
- $e) -
(zA
- Z~)~/48
(1)
The 2’s refer to the electronegativities of the atoms A4and B forming the bond. The author, however, on page 69 gives Q = 1 - e -l/r(zA-~B)* (2) which conflicts with equation 1 in making Q vary nearly with (ZA - Z B ) ~instead of with (zA- ze) for small values of zA - zB (compare the figure on page 80). Hence some of the author’s Q values and derived conclusions are questionable. Thus for C=N in R-CsN, where the bond moment indicates about 57 per cent ionic character (cf. page 75), equation 2 gives only 18 per cent, while equation 1 gives 47 per cent. T o be sure, equation 2 fits well the values 5, 11, and 17 per cent for Q for H I , HBr, and HCl deducible from observed bond moments, p , if with the author one assumes M = Qer ( r = internuclear distance). But it has been pointed out (Zoc. cit.,’ pages 684-5) t h a t fi must in general contain a large “homopolar dipole” contribution ph, here of polarity H-X+, so that p = Qer - ph, making the true Q’s much larger than those just given, and in agreement with equation 1 rather than equation 2. A related matter is the conflict between the large observedr’s of the C-I, C-Br, C-Cl bonds and the small Q values (nearly zero for C-I) predicted by the electronegativity scale. The author suggests (page 69) that these effects may be due to an unsymmetrical distribution of the electrons,-referring presumably to the homopolar dipole and related contributions,-so that the Q’s may still be as calculated. Unfortunately, the homopolar dipole terms are of polarity C-X+, whereas the bond 1 R. S. Mulliken: J. Chem. Phys. 3, 573 (1935). This paper gives (insofar as the additivity postulate is valid) a theoretical justification of the basis of the z scale.
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moments are of polarity C+X- beyond much question. There seem t o be no escape from the conclusion that all the C-X bonds have strong ionic character with the halogens negative. This indicates that the electronegativity values for the halogens require alteration, and that the electronegativity scale is less consistent and well founded than one might have hoped. There are also other indications of this: for example, the apparent absence of ionic resonance energy in the G N bond i n spite of its high polarity (footnote on page 124). Chapter IV deals with the forms of bond orbitals and their relations t o bond strengths and bond angles, and the magnetic criteria of bond type. Chapters IV and VI contain a stimulating discussion of the structure and properties of aromatic and other molecules involving resonance among several valence-bond structures. The author gives many ingenious interpretations. In the “quantitative treatment” here (pages 137-9), the large corrections in the observed resonance energies t o allow for shrinkage and stretching of bonds accompanying resonance, discussed by Lennard-Jones, might well have been mentioned. Chapter V is a thorough discussion of interatomic distances and their interpretation. Chapter VI1 contains interesting ideas on the structure of molecules with partial double-bond character. Chapter VI11 is on one-electron and three-electron bonds, including a section on semiquinones and related substances. Chapter IX is an excellent survey of the hydrogen bond problem. Chapter X deals very thoroughly with the sizes of ions and the structure of ionic crystals. Chapter X I treats “the metallic bond” rather briefly and unconventionally but interestingly. Chapter XI1 contains concluding remarks on resonance and its future applications. As was mentioned above, the book is written from the atomic orbital (AO) viewpoint. The alternative molecular orbital (MO) point of view is not discussed a t all, aside from mention without explanation on pages 138-9. Thus the unfamiliar reader will be likely to suppose that the viewpoint presented is the only possible one. This result could have been avoided by a few prefatory paragraphs concerning the MO method. Most authorities would feel that for a deeper understanding of the electronic structures of molecules a knowledge of both methods is necessary, and that for many problems the MO method is the simpler and more intelligible. For example, in problems such as that of partial ionic character, or the structure of benzene or of metals, or one-electron and three-electron bonds, where the A 0 method requires the concept of resonance among several valence-bond structures, the MO method allows one t o think in terms of a single structure. Thus one sees that “resonance” does not have the clear-cut structural significance which exclusive use of the A 0 viewpoint tends t o assign to it. Nevertheless, the use of the A 0 viewpoint makes for maximum availability of the book, especially among chemists, and the book is a landmark in the history of valence theory. In its perusal most readers will probably share the reviewer’s experience of encountering a wealth of important and interesting facts and ideas. ROBERTS. MULLIKEN.
The Cyclotron. By W. B. MANN. 92 pp. New York: Chemical Publishing Company, 1940. Price: $1.50. This timely little volume, with a brief introduction by Lawrence, treats the following subjects: magnetic resonance acceleration; cyclotron vacuum chambers and magnets; higher frequency supply and ion source; electrostatic and magnetic focusing; adjustment of the cyclotrons. I t contains a brief section on well-selected applications.
S. C. LIND.
