CHEMICAL FonmutAs. In the notation based on the atomic theory, the atoms of the several elements and their relative weights are repre sented by symbols, such as II for hydrogen, 0 for oxygen, C for carbon. etc.; H standing for 1, 0 for 16, C for 12. etc. (A list of elements, with their symbols and atomic weights, may be found in the article ATOMIC WEicirrs.) 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 disappear more or less completely. ln accordance with the assumption. the formula of a compound is made up from the symbols of its elements. For example, the formula of carbonic oxide is CO: that of carbonic acid is 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 terms of the atomic weights of its elements. For example. analysis shows that acetic acid contains 6 parts of carbon, 1 part of hydrogen, and S parts of oxygen; or—what is the same-12 parts of carbon. 2 of hydrogen, and 16 of oxygen. Using the symbol C to repre sent 12 parts of carbon, the symbol 1 part of hydrogen. and the symbol 0 16 parts of oxygen. we may denote the composition of acetic acid by the empirical formula which is nothing but a symbolic expression of the results of analysis.
But analysis also shows that certain other substances 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. Now, Avogadro's hy pothesis leads to a knowledge of the relative weight of the molecule of a compound, that weight being, namely, twice is great as the vapor density of the eompolind referred to hydrogen. See rAlotect*LEs—Mor.E.cytAa WEionTs.) So, to compare acetic acid and formaldehyde, 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 i;5 times as heavy as hydrogen. we assign to the acid the 'molecular weight' 60, and to the aldehyde the `molecular weight' 30. On the basis of this difference, we represent the t WO Coln pounds, respectively, by the formulas and Clf,,O, which have the total weights 60 and 30, m bile 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 mob cula r form u ins. In the ease of formaldehyde, the molecular formula. happens to be identical with the empirical formula. ClhO; in many other ill stances, however. this is not so. A thing ex ceedingly important to remember is, that molecu lar formulas represent practically equal volumes of substances in the yOSC0118 (or dissolved) state, with,- the scam(' pressure and tem ;•:ec AvmAmio's But even molecular formulas do not., in very many eases, suffice to characterise fully the com pounds rep WWII t •11 by them, for different 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 compound, called methyl formate or methyl formic ester, has precisely the same composition as formaldehyde and acetic acid, and precisely the same vapor density as the latter. It must therefore be represented by the same molecular formula as acetic acid: 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 assumptions of the doctrine of valency. They assume an atom of hydrogen to be always 'uni-valent,' because as a rule it is incapable of holding in combina tion more than a single atom of another ele ment; they assume an atom of carbon to be 'quadri-valent' because in marsh gas, CIL. 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, they find it combined with two 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: The differeuves 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 a•tie acid the eor•esponding oxygen atom links- a carbon atom to an atom of hydrogen. .1,in explanation of the principles used in deter mining which of all graphic formulas possible in a given ease should he assigned to the com pound under consideration may be found in the artiele t'Annox ComPoeios. To determine the `chemical constitution' of a rompound means to determine its graphic formula ; for the cor r. spends with, and is therefore a simple expres sion of. its must important Amide:11 properties.