Page:The New International Encyclopædia 1st ed. v. 02.djvu/246

ATOMIC WEIGHTS. raise the temperature one degree), a number is obtained which is nearly the same whichever ele- ment is considered. Thus the atomic weight of iron being about 56, and its specific heat 0.1138, the product is about 6.37 ; again, the atomic weight of zinc being about 65.2, and its specific heat 0.0956, the product is about 6.24. The two products, while riot absolutely the same, are near- ly equal. Why the products of such different numbers should be about the same is not yet clearly understood. But the fact can nevertheless be used for practical purposes. The mean value of the products corresponding to the several ele- ments is about 6.3. If, therefore, a new element was discovered, and it would be desirable to as- certain roughly its atomic weight, all a chemist would have to do would be to determine its spe- cific heat and divide 6.3 by that quantity. This method of determining atomic weights is espe- cially useful in eases in which there is doubt as to whether the number assigned to an element is its true atomic weight or a multiple of it. Tlie fact discovered by Dulong and Petit can thus be used at least as a cheek on the atomic weights determined by other methods.

The Periodic Law. The celebrated Russian chemist, Mendeleyeflf, demonstrated a remark- able relation between the physical and chemical properties of the elements and their atomic weights. (For an account of this relation see the article Periodic L.w. ) Suffice it to say here that the existence of this relation has definitely proved that the atomic weights at present used by chemists are in the truest sense of the term characteristic of the several elements known, and that it is hardly imaginable that future dis- covery should show them to be worthless and bring about their abolition. Once established, the Periodic Law has been employed as a check on the accepted atomic weights of the elements, but in a sense somewhat different from the law of Dulong and Petit. In several cases (e.g. in the case of platinum), accepted atomic weights were found to disagree with the general relation established by Mendeleeff. The periodic rela- tion thus indicated that those atomic weights must be somewhat incorrect : and, as a matter of fact, careful re-determination showed them to be so and brought to light their true values.

Atomic Weights Referred Either to Hydrogen OR to Oxygen. Being purely relative quantities, the atomic weights of the elements assume definite values only if referred to some fixed num- ber chosen to represent one of them. Different sets of atomic weights may therefore be obtained by choosing different standards, and one such set is obtained by assigning to the atomic weight of hydrogen the value 1. This standard seemed at one time preferable to any other, in the light of Prout's hypothesis. According to tliis the several chemical elements are not reallv differ- ent from one another in substance, but are con- densation products of hydrogen. It was conse- quently expected that careful atomic weight determination would yield whole numbers, if the atomic weight of hydrogen were taken as the unit. Experiment of the highest precision has, how- ever, shown conclusively that the atomic weights cannot be represented by commensurable num- bers; so that the common derivation of the ele- ments from one of them is at least extremely improbable, and choosing a 'common unit of measurement' for the atomic weights is as good as entirely useless. Nevertheless, the choice of a standard need not be altogether arbitrary. The actual determination of atomic weights involves analytical processes in which many of the ele- ments are weighed as oxides, the required atomic weights being calculated from the weights of these and from the atomic weight of the oxy- gen contained in them. Hence, for the sake of sim- plicitj' in precise calculations, it is desirable to have the atomic weight of oxygen represented by a whole number. Mainly for this reason chemists have chosen the atomic weight of oxygen itself as a standard, assigning to it the value 16 (the atomic weight of hydrogen is then about 1.01). Table of Atomic Weights. In the following table the first column gives the numbers recom-

Elements. Symbols. Standard, = 16. Standard, H = l. Al Sb A As Ba Bi B Br Cd Ca Ca C Ce CI Cr Co 01 Cu Er F Ga Ge Gl AU He H In I Ir Fe La Pb Li Mg Mn Hg Mo Nd Ni N Os

Pd P Pt K Pr Rh Rb Ru Sa Sc Se SI Ag Na St S Ta Te Tl Th Sn Tl W U V Tb Y Zn Zr 27.1 120. 40. 76. 137.4 208.5 11. 79.96 11-2. 133. 40. 12. 140. 36.45 52.1 59, 94. 63.6 166. 19. 70. 72. 9.1 197.2 4. 1.01 114. 126.85 193.5 56. 138. 206.9 7.03 24.36 55.0 200.3 96.0 144. 58.7 14.04 191. 16.00 106. 31.0 194.8 39.16 140. 103.0 86.4 101.7 150. 44.1 79.1 28.4 107.93 23.05 87.6 32.06 183. 127. 204.1 232. 118.6 48.1 184. 239.6 51.2 173. 89. 65.4 90.6 27.5 120. Argon 75. Barium 137. 208. 11. 80. 112. Ca?8ium 133. 40. 12. Cerium Chlorine 138. 35.5 52.6 69. Columbium 94. 63. Erbium 169. 19. 69. Glucinum Gold. . . 196.7 Hvdrocren 1. 113.4 127. 193. Iron 66. 139. Lead 207. 7. Magnesium 24. Manganese — 65. 200. 96. Nickel 68.8 14. Osmium 191. 16. 106.5 31. 197. 39. Rhodium 104. 86. 104. Samarium , 79. Silicon 28. Silver 108. 23. 87.6 32. 182. Tellurium 127. 204. Thorium 231. Tin 118. Titanium 48. 184. 240. 51.2 89. Zinc 66. 90.