Many other examples of the influence of the relative halides, since the number of halide atoms per central numbers and relative sizes of atoms or ions upon the atom may be twice the number of oxygens. stability of their compounds might be given. To menLet us summarize, a t this point, the influence of tion a few a t random: crystal structure, of ionic properties (size, valence, and 1. SFs is well known, but not SCls. SCL is the highest electronic structure), of bond type, and of the relative known chloride of sulfur, and &Bra is the highest number of electronegative elements bonded to each electropositive atom upon the strength and stability bromide known. .2. Similarly, we have MoFa, but not MoCls, MoCls of crystals as indicated by compressibility, hardness, melting point, and boiling point. being the highest molybdenum chloride known. The resistance of a crystal to compression depends An increase in volatility may be observed in a series 3. chiefly upon the interionic distances and the crystal of oxides as the relative sizes of the cations instructure, the arrangement of the building stones. If crease; but for the oxides the charge on the central the distances are large and the atoms or ions loosely atom must be six in order to have three surrounding anions, and the probability of having four screen- packed (i. e., for structures with low coordination numing anions is slight. This accounts, in part, for bers), considerable compression may be expected. Hardness is a measure of resistance to deformation. the relatively small number of gaseous oxides This is determined chiefly by the distance between known. Some interesting data are shown in atoms or ions and by the forces holding them together. Table 40. Melting point is a measure of the resistance of the crystal to disruption by heat. It is determined primarily by the strength of the bonds within the crystal. This strength, of course, is influenced by interionic disNarO MgO AhOr SiOI PIOS SO8 '32% aubl. 2800 2050 I600 sub. 250 16.8 explosive tance (in ionic crystals), valence, electronic structure CsO BaO La,% Hfa Ta?Os WOI -04 of the ions, the crystal structure, and by the relative subl, 1923 2000 2812 d. 1470 2130 130 1000 number of positive and negative elements coiirdinated to each positive atom in the crystal. The last factor In the second row, the cations are larger and the break determines, in most cases, whether or not a weak moin stability, suggesting the formation of a molecnlar lecular crystal will be formed. lattice, comes farther to the right. In both series The data in Tables 33-39 summarize the evidence the break comes further to the right than with the supporting these generalizations. ,
LITERATURE CITED
MADELUNG, H., Physik. Z., 19, 524 (1918). SHERMAN. J.. C h m . Rev.. 11, 95 (1932). VANARKEL.A. E. AND DEBOER.J. H.. "Chemisehe Bin-
(98) Go~nscwarn~, V. M., Skr. Norrke Vididens Akad. i Oslo No. 8, 1926. (99) LANCE,N. A.. "Handbook of chemistry and physics." Chemical Rubber Publishing Company. Cleveland, Ohio,
414 (1932). PAULING, L.,J. Am. Chern. Soc., 54, 3573 (1932). B ~ G M A P. N ,W., 2. Krisl., 67, 363 (1928). PAULING,L.,Z. Krist., 69, 35 (1928).
10.?4. F~;A&,
K., Z. Krirt., 61, 41 (1925). PAULING, L., J. Am. Chem. Soc., 50, 1036 1928) PAULINO, L., J. Am. Chem. Soc.. 54, 988 11932): KDSSEL,W., Ann. Physik., 49,229 (1916). GRIMM.H. G., "Handbucb der Physik," %I. 24, Geiger und Scheel, Berlin, 1927, p. 543. N. V., "The covalent link in chemistry," Cor(105) SIDGWICK. nell University Press, Ithaca, New York, 1933, p. 59. L. 0..AND PAULIND,L., PIOC.Natl. A c d . (106) BROCKWAY, Sci.. 19, 68 (1933).
(100) (101) (102) (103) (104)
INTERNATIONAL EXHIBITION OF APPLIED AND SCIENTIFIC PHOTOGRAPHY TO BE HELD IN ROCHESTER, MARCH, 1937 An International Exhibition of Applied and Scientific Photography will be held in Rochester in March, 1937, under the spansorship of the Rochester Scientific and Technical Section of the Photographic Society of America. The objective of the exhibition will be to show examples of the application of photography to the wrious branches of science and technology. The following photography, sstronomy and sections havebeen orgmieed: metrology, aerial photography, photomi~rograpby, medical photography, X-ray in industry, documentary photography, high speed photography, stereo-photography, photography in
physics and chemistry, photographic sensitivity, n a t d history, andmiscellaneous. showing the of photogphotograph or apparatus raphy to typical problems in any branch of science and technology will be welcomed. All comespondence regard to the Exhibition, or requests for entry blanks, should be addressed to the Secretary, C. B. Nehlette, F.R.P.S., Department of Photov p h i c Technology, Rochester Athenaeum and ~ e c h a n i c s1nstitute, Rochester, New York.
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