Comments on the Paper by A. D. WalshAuthor(s): J. W. LinnettSource: Proceedings of the Royal Society of London. Series A, Mathematical and PhysicalSciences, Vol. 207, No. 1088, A Discussion on Bond Energies and Bond Lengths (Jun. 7, 1951),pp. 30-31Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/98672 .

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30 A. D. Walsh (Discussion Meeting)

whose slope (ak/lU) decreases with decreasing electronegativity of A. Torkington (1948) has earlier found evidence to show that the slope (ak/Ul) of a graph of 1(B) against ke(A-B) for AB4 molecules, where A is a fixed group IV element and B is a halogen atom, decreases with decreasing electronegativity of A.

The behaviour shown on figure 7 is not limited to the case where A is a group IV element. Figures 8 and 9 show the corresponding plots for elements in groups I and III. In each case surprisingly good straight lines are obtained, especially where four points can be plotted for a particular element.

The linearity of the plots in figures 6 and 7 suggests trying a plot of ke(A-B) against ke(A-B') where A is a group IV element, B is a group VI element and B' is the group VII element next to B in the Periodic Table. Figure 10 does this. A straight line is obtained which passes through the origin. In other words, the force constant of a diatomic molecule formed between a group IV element and a group VI element, B, is directly proportional to the force constant of the molecule formed between the same group IV element and the group VII element of atomic number greater by unity than that of B.

The author is indebted to Dr J. W. Linnett and Dr H. A. Skinner for valuable comments.*

COMMENTS ON THE PAPER BY A. D. WALSH

BY J. W. LINNETT

I agree very much with Dr Walsh that a satisfactory knowledge of effective atomic electronegativities is very important and that we should concentrate on obtaining such knowledge. The effective electronegativity of an atom varies, of course, in different circumstances. This appears to be the explanation of the difference in strength (length and force constant) of the C-H bonds in methane and acetylene. The effective attraction of the carbon atom for the electrons in a C-H bond in methane is a resultant of the attraction of the carbon nucleus plus is core and the repulsion of the electrons associated with the other three C-H bonds. In acetylene the effective attraction of the carbon atom for the electrons in the C-H bond is the resultant of the attraction of the nucleus plus core and the repulsion of the electrons in the triple bond. Because these last six electrons are drawn away into the triple

* (Note added in proof.) In speaking of previous definitions of electronegativity, the author regrets that he over-

looked a paper by Gordy ( 946 c). Since the present paper was read, further data have become available. The work of Porter has led to the value kCe = 4' 10 x 105 dynes/cm. for the SH radical: this has been added to table 7. The point for sulphur lies, as expected, a little below the curve of figure 3. A second edition of the book referred to as Herzberg (1939) revises earlier and tabulates new values of t0e. From this, values of ke for InH and SnH have been added to table 7. A new k, value for AIF does not fit the linear relation of figure 9. Values for GeF, GeCl, GeBr, however, fit well a linear relation as in figure 7. According to new data of Hultin & Lagerqvist (1950), the value for CaO in table 6 should be raised to 3-61.

Torkington showed that the plot of ke(CH3--Hal.) against I(Hal.) was linear but of smaller slope (U2/0I) than that of ke(H-Hal.) against I(Hal.). In conjunction with the behaviour revealed by figures 6, 7, 8 and 9 this may be taken as evidence that X(CH3) <%(H).

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Comments on the paper by A. D. Walsh 31

bond on the far side of the carbon atom from the C-H bond it seems that their repulsion on the electrons in the C-H bond is less than that of the six electrons in the three other C-H bonds in methane which are relatively nearer. The effective electro- negativity of the carbon atom (with its attendant electrons) is therefore greater in acetylene than in methane. Therefore the C-H bond is shorter and has a bigger force constant in acetylene than in methane. Also because the electrons of the C-H bond in acetylene are more firmly held to the carbon atom the hydrogen is more readily lost as a proton without the electrons of this bond, which are left on the carbon, and so the hydrogen in acetylene is somewhat acidic in character and the acetylides are formed.

In many cases ionization potential (or some quantity dependent on it) has been used as a measure of electronegativity. Dr Walsh has, moreover, shown that a corre- lation exists between the force constants of the MH molecules and the ionization potentials of the atoms M. It seems to me that some care should be used in the choice of the ionization potential that should be used. When an electron is lost from Li, B, C or N the number of unpaired electrons decreases. When an electron is lost from Be, O or F the number of unpaired electrons increases because an electron that was originally paired with another is lost. Now the operative electron in forming a bond is that which is unpaired. Consequently we ought to use as the ionization potential to be related to properties of bonds that corresponding to the loss of an electron that is involved in bonding. For instance let us take the fluorine atom as an example. The structure of this is 1822822p5 with one unpaired electron which is the valency electron. The ordinary ionization potential is that corresponding to the removal of an electron to give F+ with an electronic structure of 1s22s22p4 with two of the 2p electrons having their spins paired and the other two having parallel spins so that the resultant spin quantum number is unity. But really we should consider, for correlation with valency properties, the energy for the removal of a valency electron. This process would leave lS22s22p4 in which the 2p electrons consist of two pairs with the resultant spin quantum number equal to zero. The difference between the ionization potential that should be used and the ordinary ionization potential may be considerable. Similar considerations apply to the other halogens and to oxygen and its analogues. The case of beryllium and its analogues is rather different because one may regard the ground state of Be as being different from the valency state and, in this case, changes of a rather different kind would have to be applied.

COMMENTS ON THE PAPER BY A. D. WALSH

BY H. A. SKINNER

Theoretical justification for a relationship between ionization potential (I) and electronegativity (X) rests in the equation derived by Mulliken (I934, I949), viz.:

= +2 where E is the electron affinity. Elements situate on the left-hand side

of the Periodic Table have small E values, so that in these cases a direct relationship between X and I might be anticipated. On the right-hand side of the Periodic Table,

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