August 16, 1966

Dr. D. F. C. Morris

Department of Chemistry Brunel

University

Woodland Avenue, Acton W. 3

London, England

Dear Dr. Morris:

I was pleased to receive your letter, and to learn about the papers by Fumi and Tosi.

I have not seen these papers, but I remember having had some correspondence with Fumi about ionic radii. I do not remember the details of the correspondence.

I must say that I do not see any reason to change the values of ionic radii significantly from those given in my book, which I had derived in 1927, and which agrees with those of Goldschmidt and Wasastjerna. The values given in 1923 by Wasastjerna were derived essentially by the method that you describe, in your letter, I think that Wasastjerna started with the assumption that lithium ion has very small polarizability.

The method that I used is somewhat more refined. In my 1927 paper in the Proceedings of the Royal Society (ref. p. 513) I showed that different screening constants might be used in the discussion of different properties. I derived some screening constants by theoretical calculation and some by the interpretation of ionic polarlzabillties and X-ray energy levels. Then by using the screening constants appropriate to ionic size, I obtained ionic radii for alkali ions and halogenide ions.

As a rough approximation, the polarizability may be assumed to be proportional to the ionic volume. The index of refraction of a crystal cannot be used in any completely consistent way to obtain values of polarlzabilies of individual ions, because the outer electrons are attracted by more than one nucleus. It is interesting that your application of the Wasastjerna method to the 1953 values of ionic polarizabilities leads to values of the ionic radii agreeing with my values to within about 0.01A, on the average, whereas Wasastjerna’s values agreed within about 0.05A.

I think that it would be very hard to justify the use of the Wasastjerna method, except as a rough approximation.

The theory that I developed in 1926 and 1927 is still valid, and I do not know about any other theoretical treatment of the problem that seems to me to be better. I may point out that in my derivation of values of ionic radii I did not make any use of the experimental values of polarizabilities of ions (except of the 18-shell ions).

From time to time I have made an effort to improve my earlier theory, but so far without result.

Sincerely yours,

Linus Pauling

Signed by Dr. Pauling's secretary

LP:fkg