(7) A compound of N and H gas weighs 0.765 g. to the liter. The N is 14/17 of the whole, the H 3/17. What gas is it? CHAPTER XXXV. ATOMIC WEIGHT. 174. Definition.--We have seen that the molecular weight of a compound, as well as of most elements, is obtained from the vapor density by doubling the latter. It remains to explain how atomic weights are obtained. The term is rather misleading. The atomic weight of an element is its least combining weight, the smallest portion that enters into chemical union, which is, of course, the weight of an atom. 175. Atomic Weight of Oxygen.--Suppose we wish to find the atomic weight of oxygen. We must find the smallest proportion by weight in which it occurs in any compound. This can only be done by analyzing all the compounds of O that can be vaporized. As illustrative of these compounds take the six following:-- Wt. of other Names. V. d. Mol. Wt. Wt. of O. Elem. Symbol. Carbon monoxide... 14 28 16 12 ? Carbon dioxide.... 22 44 32 12 ? Hydrogen monoxide... 9 18 16 2 ? Nitrogen monoxide... 22 44 16 28 ? Nitrogen trioxide... 38 76 48 28 ? Nitrogen pentoxide... 54 108 80 28 ? 176. Molecular Symbols.--From the vapor density of the gases-- column 2--we obtain their molecular weight-- column 3. To find the proportion of O, it must be separated by chemical means from its compounds and separately weighed. These relative weights are given in column 4. Now the smallest weight of O which unites in any case is its atomic weight. If any compound of O should in future be found in which its combining weight is 8 or 4, that would be called its atomic weight. By dividing the numbers in column 4, wt. of O, by 16, the atomic weight of O, we obtain the number of O atoms in the molecule. Subtracting the weights of O from the molecular weights, we have the parts of the other elements, column 5, and dividing these by the atomic weight of the respective elements, we have the number of atoms of those elements, these last, combined with the number of O atoms, give the symbol. In this way complete the last column. Show how to get the atomic weight of Cl from these compounds, arranging them in tabular form, and completing as above: HCl, KCl, NaCl, ZnCl2, MgCl2; the atomic weight of N in these: N2O, NO, NH3.