Page:The New International Encyclopædia 1st ed. v. 04.djvu/645

* CHEMISTRY. J63 CHEMISTRY. speak a lanjriia.se free from the atomic or any other hypothesis, and based on notliing but ex- j)erinicutal observations. CuEJliCAi. KoKiii'i.AS. In the notation based on the atomic theory, the atoms of the several elements and their rehitive weights are repre- sented by symbols, such as H for liydrogen, O for oxvgen, C for carbon, et<?. ; H standing for 1, O for" IG, C for 12, etc. (A list of elements, with their symbols and atomic weights, may be found in the article Atomic YEiGiir.s.) When two or more elements combine chemically, their atoms are assumed to become associated in groups (molecules) without being in any way changed. The assumption is based on the fact that the elements of a compound can be re- obtained from it in the free state, though we do not know, of course, what really becomes of an element when it combines with other ele- ments; for. as we have seen above, chemical com- bination usually causes the properties of the ele- ments to (lisap])ear more or less completely. In accordance with the assuniption, the formvila of a compound is made up from the sjnnbols of its elements. For example, the formula of carbonic oxide is CO: that of carbonic acid is CO,, etc.; C denoting one atom of carbon, 0; two atoms of oxygen, etc. The formulas at present used by chemists are of three different kinds — viz. em- pirical, molecular, and graphic. An empirical formula, as the name suggests, may be considered as involving no hypothesis whatever; it is merely the simplest form in which the composition of a compound may be expressed in tenns of the atomic weights of its elements. For example, analysis shows that acetic acid contains fl parts of carbon, 1 part of hydrogen, and 8 parts of oxj'gen; or — what is the same — 12 parts of carbon, 2 of hydrogen, and 10 of oxygen. Using the symbol C to repre- sent 12 parts of carbon, the sjTnbol H 1 part of hydrogen, and the symbol O 16 parts of oxygen, we may denote the composition of acetic acid by the empirical formula CH.O, which is nothing but a symbolic expression of the results of analysis. But analysis also shows that certain other substanc-es have the same composition as acetic acid ; for example, the well-known formaldehyde. It is therefore clear that, in order to denote their compounds in a definite manner, chemists must employ formulas which express something else besides composition. Mow, Avogadro's hy- pothesis leads to a knowledge of the relative weight of the molecule of a compound, that weight being, namely, twice as great as the va])or density of the compound referred to hydrogen. (See Molecules — Molecular Weights.) So, to compare acetic acid and foimaldehyde, we deter- mine their vapor densities, and as the vapor of acetic acid is found to be 30 times as heavy as hydrogen, and formaldehyde vapor is found to be 15 times as hea'' as hydrogen, we assign to the acid the 'molecular weight' fiO, and to the aldehyde the 'molecular weiglit' 30. On the basis of this difference, we represent the two com- pounds, respectively, by the formulas C-HjOc and CH.O, which have "the" total weights HO and 30, while the relative weights of the constituent elements are obviously the same in both. For- mulas like these, which denote not only the relative composition of substances, but also their molecular weights, are termed molecular formu- las. In the case of formaldehyde, the molecular formula, C'lLO, happens to be identical with the empirical formula, CIL.O; in many other in- stances, however, this is not so. A thing ex- ceedingly important to remember is, that molecu- lar formulas represent practieallij equal volumes of substances in the gaseous ( or dissolved ) state, under the same conditions of pressure and tem- perature. See AvooADno's Kile. But even molecular fornuilas do not, in vcrj' many cases, sufiice to characterize fully the com- pounds represented by them, for dillerent com- pounds may have not only the same composition, but also the same molecular weight. Consider, for example, the ethereal salt formed by the action of formic acid on wood alcohol. This compoimd, called methyl formate or methyl- formic ester, has precisely the same composition us formaldehyde and acetic acid, and precisely the same vapor density as the latter. It must therefore be represented by the same molecular formvila as acetic acid: C,H,0, C,lI,Oj Acetic acid Methyl formate To differentiate compounds like these, chemists use graphic or structural formulas. Such for- mulas represent compounds as different because the atoms are differently combined within their molecules, though the kind and number of atoms are the same. To exhibit these differences of combination, chemists employ the assunijitions of the doctrine of valency. They assume an atom of hydrogen to be always 'univalent,' because as a rule it is incapable of holding in combina- tion more than a single atom of another ele- ment ; the' assume an atom of carbon to be ■quadri-valent' because in marsh gas, CH,, they find it combined with four atoms of hydrogen, and for other reasons of the same nature; and they assume an atom of oxygen to be 'di-valent' because in water, H,0, they find it combined with tico atoms of hydrogen. With the aid of these assumptions, symbolized by dashes ('bonds') linking together the atoms, they rep- resent acetic acid and methyl formate, respec- tively, by the following graphic formulas: " H H I I O H— C— H 0=C O H— C-H 0=C I I H H Acetic acid Methyl formate The differences between the two formulas are obvious ; thus, the formula of methyl formate shows one of its oxygen atoms as linking to- gether two carbon atoms, while in the formula of acetic acid the corresponding oxygen atom links a carbon atom to an atom of hydrogen. An explanation of the principles used in deter mining which of all graphic fornuilas possible in a given case should bo assigned to the com- pound under consideration may be foiiiil in the article Cauhox Compounds. To determine the 'chemical constitiitiim' of a compound means to determine its graphic formuhi : for the latter cor- responds with, and is therefore a simple expres- sion of, its most importairt chemical properties. fJraphic formulas often have an abbreviated