Atomic Weight

Atomic Weight (see Atom), the relative weight of atoms of chemical elements, with the weight of the lightest element, hydrogen, taken as the unit. The determination of atomic weight became possible after Avogadro's hypothesis, according to which equal volumes of gases contain equal numbers of molecules under the same conditions of temperature and pressure, i.e., the molecular weights of gaseous substances are proportional to their densities. Taking the weight of a hydrogen molecule as 2 (as indicated by the volumetric ratios in which hydrogen combines with other gases), one can find the molecular weight of any gaseous substance from its density. If, for example, oxygen is taken, analysis of its various compounds shows that in a quantity of grams equal to their molecular weight (in gram-molecules), one finds either 16 g of oxygen or a whole multiple of this number; consequently, 16 is the atomic weight of oxygen. The relationship between heat capacity and atomic weight of solid elements, discovered by Dulong and Petit, as well as the phenomena of isomorphism and the periodic system of elements established by Mendeleev, were also of great importance for determining atomic weights. Among modern methods for determining atomic weight, one should mention a) purely chemical methods, in which the weight relationships between elements in a compound are determined by analysis (the formulas of the compounds are assumed to be already known); b) the method of determining atomic weight by Aston, based on the study of the deviation of canal rays in electric and magnetic fields. Since in most cases it is necessary to use oxygen compounds of elements when determining atomic weights, it is more convenient in practice to relate these weights not to the hydrogen unit but to the oxygen unit. For this purpose, the oxygen atom was conventionally assigned the value O = 16.00; with such a unit, the atomic weight of hydrogen, according to the latest data, is H = 1.0078. The atomic weights of elements Na, K, Ag, Cl, Br, J, H, C, N, S have been determined by the most varied methods with special care; they are called fundamental atomic weights, and the atomic weights of other elements are determined by comparison with them. In view of the particularly important significance of atomic weights for chemistry, an international atomic commission was formed in 1900, which annually publishes critical reports on work in this field and accordingly revises the tables of atomic weights. As a result of the imperialist war, this commission split into a German commission and a commission of the anti-German coalition. Below is a table of atomic weights (as of 1927 according to the German commission). Ag Silver. Manganese. 54.93 Al Aluminum Mo Molybdenum 96.0 Nitrogen 14.008 As Arsenic Na Sodium 23.00 Au Gold Nb Niobium 93.5 B Bor Nd Neodymium 144.3 Ba Barium Ne Neon 20.2 Be Beryllium Ni Nickel 58.68 Bi Bismuth Cl Chlorine 16.000 Br Bromine Os Osmium 190.9 C Carbon P Phosphorus 31.04 Ca Calcium Pb Lead 207.2 Cd Cadmium Pd Palladium 106.7 Ce Cerium Pr Praseodymium 140.9 Cl Chlorine Pt Platinum 195.2 Co Cobalt Ra Radium 226.0 Cs Cesium Rh Rhodium 102.9 Cu Copper S Sulfur 32.07 Dysprosium Sb Antimony 121.8 Em Emanation Sc Scandium 45.10 Er Erbium Se Selenium 79.2 Europium Si Silicon 28.06 F Fluorine Sm Samarium 150.4 Fe Iron Sn Tin 118.7 Ga Gallium Sr Strontium 87.6 Gd Gadolinium Ta Tantalum 181.5 Ge Germanium Tb Terbium 159.2 Hydrogen Te Tellurium 127.5 Helium Th Thorium 232.1 Hafnium Ti Titanium 48.1 Mercury Tl Thallium 204.4 Holmium Tm Thulium 169.4 Indium Uranium 238.2 Iridium Vanadium 51.0 Iodine Wolframium 184.0 Potassium Xenon 130.2 Krypton Yttrium 89.0 Lanthanum Ytterbium 173.5 Lithium Zinc 65.37 Magnesium Zirconium 91.2 Lit.: Mendeleev D.I., Principles of Chemistry, M.-L., 1927; Ostwald W., Outline of General Chemistry, 6th ed., 1920; on the determination of atomic weights see Brauner's article in (Abegg and Auerbach, Handbook of Inorganic Chemistry, Lpz., 1903-27.) I. Kazarovsky.