Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/484

Rh 472 CHEMISTRY [NOTATION. rapidly increases. These two elements, in fact, afford striking examples of the resolution of complex molecules into simpler molecules by heat; and it is by no means improbable that the tetratomic phosphorus and arsenic molecules will be found to behave similarly if sufficiently heated. No method is known by which it is possible to determine the molecular weights of elements or compounds in any other state than that of gas, and the behaviour of sulphur is alone sufficient to prove that we are not justified in assuming that the molecular weights of liquid or solid elements or compounds are identical with their molecular weights as gases. Indeed it is in the highest degree probable that the molecules of solid and liquid bodies are very frequently far more complex than the molecules of the same bodies in the state of gas. Chemical Notation Valency of Elements Rational Formulae. Opposite the name of each element in the second column of the table on p. 467, the symbol is given which is always employed to represent it. This symbol, however, not only represents the particular element, but a certain definite quantity of it. Thus, the letter H always stands for 1 atom or 1 part by weight of hydrogen, the letter N for 1 atom or 14 parts of nitrogen, and the symbol Cl for 1 atom or 35 - 3 6 parts of chlorine. Compounds are in like manner represented by writing the symbols of their constituent elements side by side, and if more than one atom of each element be present, the number is indicated by a numeral placed on the right of the symbol of the element either below or above the line. Thus, hydrochloric acid is represented by the formula HC1, that is to say, it is a compound of an atom of hydrogen with an atom of chlorine, or of 1 part by weight of hydrogen with 35 36 parts by weight of chlorine ; again, sulphuric acid is represented by the formula H 2 S0 4, which is a statement that it con sists of 2 atoms of hydrogen, 1 of sulphur, and 4 of oxygen, and consequently of certain relative weights of these elements. A figure placed on the right of a symbol only affects the symbol to which it is attached, but when figures are placed in front of several symbols all are affected by it, thus 2H 2 S0 4 means H 2 S0 4 taken twice. The distribution of weight in chemical change is readily expressed in the form of equations by the aid of these symbols ; the equation for example, is to be read as meaning that from certain definite weights of hydrochloric acid and zinc certain definite weights of two different bodies, zinc chloride and hydrogen, are produced. The + sign is invariably employed in this way either to express combination or action upon, the meaning usually attached to the use of the sign = being that from such and such bodies such and such other bodies are formed. Usually, when the symbols of the elements are written or printed with a figure to the right, it is understood that this indicates a molecule of the element, the symbol alone representing an atom. Thus, the symbols H 2 and P 4 indicate that the molecules of hydrogen and phosphorus respectively contain 2 and 4 atoms. Since, according to the molecular theory, in all cases of chemical change the action is between molecules, such symbols as these ought always to be employed. Thus, the formation of hydrochloric acid from hydrogen and chlorine is correctly represented by the equation H 2 + C1 2 = 2HC1; that is to say, a molecule of hydrogen and a molecule of chlorine give rise to two molecules of hydrochloric acid ; whilst the following equation merely represents the relative weights of the elements which enter into reaction, and is noi a complete expression of what is supposed to take place : = HC1 As the molecular weights of comparatively few of the elements have been determined, however, it is possible only in a limited number of cases to employ such symbols. The molecular weights of the larger number of compounds are also unknown, but in all cases it is usual to represent them by formulae which to the best of our know ledge express their molecular composition in the state of gas, and not merely the relative number of atoms which they contain; thus, acetic acid consists of carbon, hydrogen, and oxygen in the proportion of one atom of carbon, two of hydrogen, and one of oxygen, but the determination of its vapour density shows that it has a molecular weight corresponding to the formula C 2 H 4 O 2, which therefore is always employed to represent acetic acid. When chemical change is expressed with the aid of molecular formulae, hot only is the distribution of weight represented, but from mere inspection of the symbols it is possible to deduce the relative volumes which the agents and resultants occupy in the state of gas if measured at the same temperature and under the same pressure. Thus, the equation 2H 2 + O 2 = 2H 2 O not only represents that certain definite weights of hydrogen and oxygen furnish a certain definite weight of the compound which we term water, but that if the water in the state of gas, the hydrogen, and the oxygen are all measured at the same temperature and pressure, the volume occupied by the oxygen is only half that occupied by the hydrogen, whilst the resulting water-gas will only occupy the same volume as the hydrogen. In other words, 2 volumes of oxygen and 4 volumes of hydrogen furnish 4 volumes of water gas. A simple equation like this, there fore, when properly interpreted, affords a large amount of in formation. One other instance may be given; the equation represents the decomposition of ammonia gas into nitrogen and hydrogen gases by the electric spark, and it not only conveys the information that a certain relative weight of ammonia consisting of certain relative weights of hydrogen and nitrogen is broken up into certain relative weights of hydrogen and nitrogen, but also that the nitrogen will be contained in half the space which contained the ammonia, and that the volume of the hydrogen will be one and a half times as great as that of the original ammonia, so that in the decomposition of ammonia the volume becomes doubled. Formulas which merely express the relative number of atoms of the different elements present in a compound are termed empirical formulae, and the formulae of all com pounds whose molecular weights are undetermined are necessarily empirical. The molecular formula of a com pound, however, is always a simple multiple of the empi rical formula, if not identical with it; thus, the empirical formula of acetic acid is CH 2 O, and its molecular formula is C 2 H 4 O 2, or twice CH 2 0. If the vapour density of a compound cannot be ascertained, its molecular formula can only be ascertained, with .more or less approximation to truth, by considering its properties, and especially its rela tions to allied compounds of known molecular weight. For example, chromic anhydride is represented as CrO 3 , and, although it cannot be vaporized, this is held to be. its molecular formula chiefly on account of the formation from chromic anhydride of a volatile chromium oxychloride, the molecular weight of which is found to accord with the formula Cr0 2 Cl 2. But in addition ^ empirical and molecular formulae,