NEW BOOKS
1107
NEW BOOKS General Chemistry. A n Introduction to Descriptive Chemistry and jCfodern Chemical Theory. By LINUSPAULINQ. vii 595 pp.; 117 figs. San Francisco, California: W.H. Freeman and Company, 1947. Price: 54.25. A general chemistry text by Linus Pauling deserves special attention. I t is not an exaggeration to say that his The Nature of the Chemical Bond,first published in 1939, has exerted a greater influence on chemical literature and the beliefs of many chemists than any other recent book. The presentation in this book, which was thought t o pioneer a broad application of Heisenberg’s concept of wave mechanical resonance t o the problem of electronic structure of molecules, was deliberately one-sided, and the proposed resonance structures were necessarily and admittedly arbitrary. On the other hand, an introduction to chemistry should be devoted primarily to well-established facts and principles t o which chemistry owes its place among the exact sciences and should distinguish clearly between such principles and “useful” or “convenient” hypotheses, rules, and pictures. One might wish that the author had used this occasion to test critically which aapects of his approach have proved to be sufficiently well founded to justify their inclusion in an elementary text. The following statements (p. 2) were promising in this respect: “A special effort baa been made in this book t o present the subject of chemistry in a logical and simple manner decide on the applicable principle and get it clearly in mind. Then apply i t in a straightforward way.” The reviewer undertook to find out whether this text enables the beginning student or even an experienced teacher to decide which reliable principles must be applied, especially in treating problems of chemical binding and the electronic structure of molecules. It was very disappointing t o find that most of the information involved has not been correlated in a logical and simple manner with the general principles of energy, electric forces, and quantum states, but is either based on rules which the reviewer considers inadequate or is given in the form of authoritative assertions. Only a few examples can be given here asillustrations. Pauling considers (pp. 141 and 165) the most important foundation of electronic formulations of covalently bonded molecules t o be G. N. Lewis’ postulate that an atom has the tendency to achieve a noble gas configuration and his suggestion that the shared electron pair is completing the shell of each of the bound atoms. While opinions are divided as t o the soundness of this suggestion, one finds in The General Chemistry many well-known examples which clearly cdntradict the general validity of the postulate. Following the prwentation of Lewis’ electronic structures of the four halogen molecules X1 (p. 147) and of the ring molecule S, the chain molecule of sulfur is characterized (p. 148) as having two end atoms of “abnormal” structure (four unshared and two shared electrons). On p. 149, 02 is said t o have “an unusual electronic structure. We might expect this molecule to contain a double bond; i.e., t o share two electron pairs between the two atoms, and thus t o complete the octet for each atom. Instead only one shared pair is formed, leaving two unpaired electrons which are responsible for the paramagnetism of oxygen.” The
+
.
..
certainty with which the formula :O:O:, indicating a septet around each atom, is postulated
.. .
in the text is puzzling in view of the information given in a footnote, that the actual bond strength is larger than expected for a single bond and that the oxygen molecule may have a “special sort” of bond, :0-0: . The latter structureseems to indicatea ten-electron configuration around each atom. Thus it is evident that the oxygen atom has no tendency to form either an octet or a double bond with another oxygen atom. Contrary to this conclusion, it is said (p. 150)that ozone “has the electronic structure”
.. *.
..
.. ..
: O : O : : O ,in which the central atom is connected with one end atom by a single bond and with the other by a double bond, all atoms completing their octets by sharing. However, in order to achieve this formal agreement with the octet rule a new complication has t o be
1108
NEW BOOKS
admittcd. “Two structures arc shown above, in brackets.” (p. 150) The reason is that “it is known fromexpcriment that the two oxygen-oxygen bonds in ozone are not different, but Zquivalence of the bonds can be explained (?) by the assumption of a hybrid structure. Each of the bonds in ozone is a hybrid between a single bond and a double bond, and its propertics are intermediate.” (p. 155) A student not acquainted with the allcged wave mechanical justification for this artificial attempt t o overcome the inadequacies of the valence bond theory would conclude t h a t the properties of a bond intermediate between a two-electron and a four-electron bond are those of a three-electron bond. Hence he may wonder why ozone is not Paramagnetic, as is the oxygen molecule pictured above with two three-electron bonds. The writing of multiple resonance structures, applied abundantly in the General Chemistry, will be misleading in the following respect also. The statement t h a t they do not correspond t o different kinds of molecules seems t o be mentioned only on pp. 155 and 505, in connection with ozone and benzene, rcspectively. Thus, having been authoritatively told on many other occasions thnt the given resonance formulas represent the electronic structure of the molecules involved, the reader is not sufficiently protected against the belief that each of these structures has a physical significance. This belief is further strengthened by statements such as “the double bond resonates between the two positions in ozone” (p. 155) or even “moves around” (p. 152)in the rings of graphite. No more appropriat,e for inclusion in an introductory text than the resonance structures is the electronegativity scale of elements, considered t o be so important that its numerical values are given in two tables (without designation of the units) on pp. 160 and 544. These values, ranging between 0.7 for Cs and 4.0 for F, are said t o represent the elements’ “power of attraction for the rlcct,rons in a covalent bond.” Since t,hese values are applied in the General Chemistry and The Nature of the Chemical Bond also t o typically ionic substances, the emphasis on a covalent, bond is not clear and in addition the word “elements” should probably be replaced by “atoms.” Furthermore, this definition is based on the assumption t h a t the relative polarity of two atoms, A and B, is independent of the compound in which they nre bonded. Howevcr, the polarity in the C+Br direction is generally considered t o be
0
+ - in a n acyl bromide, RC+Br,
but - f in the acetylene derivative CH,(CC)+Br. Even in the GeneraZChernistry, in six compoundscontaininga nitrogen-oxygen “bond,” the oxidation number or st,ate of nitrogen varies (p, 317) between -1 in H2NOH and +5 in K ~ O S with respect t o -2 of oxygen. Thus the vague concept of a constant electronegativity of an element cannot have a definite meaning even in qualitative respects. The above examples do not support the belief t h a t the concepts of octet formation by the sharing of electrons, resonance, and electronegativity should be included among those “unifying theoretical concepts” which, according t o the preface, make the contemporary presentation of general chemistry more “simple, straightforward and logical” than t h a t in former times. Space available for a book review does not permit evaluating the treatment in the General Chemistry of other aspects of molecularstructure. I t can also onlybe mentioned t h a t the presentation of better-established subjects has, despite weaknesses, unquestionable merits, Some of these weaknesses have been removed in College Chemistry, written by tho same author and published by W . H . Freeman and Company in 1950. However, the topics discussed above are presented in the newer book in exactly the same way as in the General Chemistry. Hence this review is timely, particularly in connection with a recent discussion (J. Am. Chem. Soc. 72.4335 (1950)) of the foundation on which the resonance formulations used in these books are based. KASIUIRFAJANS.
The Physical Chemistry of Dyeing. By T. VICKERSTAFF. 416 pp. London: Oliver and Boyd. Price: 42 8 . net. I n spite of the past twenty years’ intensive research into the chemical mechanism of the dyeing process, much of it has been available only in original papers scattered throughout
