That is to say, the ratios are not disturbed by the assumption that in marsh gas we have I atom of hydrogen combined with I atom of carbon, having the relative combining weight of 3, and in ethylene I atom of hydrogen united with 2 atoms of carbon. It will be evident, however, that if we could gain any exact information as to the actual number of atoms which are present in these various molecules, this difficulty would no longer exist. For example, suppose it were possible to ascertain that in the molecule of marsh gas there were 4 atoms of hydrogen, then as the relative weights of hydrogen and carbon in this compound are as I 3, the weight of the carbon atom would obviously have to be raised from 3 to 12; and if it could be determined that in the ethylene molecule there were also 4 atoms of hydrogen, then seeing that the ratio of hydrogen to carbon in this substance is as 1 : 6, we should conclude that it contained 2 atoms of carbon, of the relative weight not less than 12, and the composition of the two compounds would be expressed by the formulæ, marsh gas CH, ethylene CH4. But Again, the relative weights of hydrogen and oxygen in water are as 8. If the molecule of water contains only 1 atom of hydrogen, then we conclude that 8 represents the relative weight of the oxygen atom, and the formula for water will be HO. suppose it to be discovered that there are two atoms of hydrogen in a molecule of this compound, then it becomes necessary, in order to retain the ratio between the weight of these constituents (a ratio ascertained by analysis), to double the number assigned to the oxygen atom and to regard its weight as 16, as compared with I atom of hydrogen, and the formula for water in this case would be H2O. The compound ammonia contains the elements hydrogen and nitrogen in the ratio Hydrogen nitrogen: : : 4.66. If the molecule of ammonia contains only 1 atom of hydrogen, then 4.66 represents the relative weight of the nitrogen atom, and the formula will be NH; but if it should be found that there are 3 atoms of hydrogen in this molecule, then again the relative weight assigned to the nitrogen must be trebled in order to preserve the ratio, and it will have to be raised from 4.66 to 14 (in round numbers), and the formula for ammonia will be NH3. From these considerations it will be evident, that it is of the highest importance to gain accurate knowledge as to the actual number of atoms which are contained in the molecules of matterin other words, to learn the true atomic composition or structure of molecules; and it may be said that this problem has occupied the minds of chemists from the time that Dalton published his atomic weights, in the year 1808, down to the present time. There is no single method of general application, by means of which chemists are able to determine the atomic weight of an element ; but they are guided by a number of independent considerations, some of which are chemical in their character, while others are of a physical nature; and that particular number which is in accord with the most of these considerations, or with what are judged to be the most important of them, is accepted as the true atomic weight. The chief methods employed for determining atomic weights may be arranged under the following four heads : 1. Purely chemical methods. 2. Methods based upon volumetric relations. 3. Methods based upon the specific heats of the elements. I. As an illustration of the chemical processes from which atomic weights may be deduced, the following examples may be given, namely, the case of the two elements oxygen and carbon. Oxygen combines, as already stated, with hydrogen in the proportion Hydrogen oxygen = 1:8. When water is acted upon by the element sodium, the compound is decomposed and hydrogen is evolved; and it is found that if 18 grammes of water are so acted on, I gramme of hydrogen is evolved, and 40 grammes of a compound are formed, which contains sodium, together with all the oxygen originally in the 18 grammes of water, and some hydrogen. This compound, under suitable conditions, can be acted upon by metallic zinc, and when these 40 grammes are so acted on, 1 gramme of hydrogen is again evolved, and 72.5 grammes are obtained of a compound containing no hydrogen, but sodium and zinc combined with all the oxygen originally contained in the 18 grammes of water. It will be evident, therefore, that the hydrogen contained in water can be expelled in two equal moieties; there must, therefore, be two atoms of hydrogen in this compound. By no known process can the oxygen be withdrawn from water in two stages: thus, if 18 grammes of water are acted upon by chlorine, under the conditions in which chemical action can take place, 73 grammes of a compound containing only chlorine and hydrogen are formed, and the whole of the oxygen is thrown out of combination and evolved as gas. It is therefore concluded that water contains in its molecule 2 atoms of hydrogen and I atom of oxygen, and as they are combined in the relative proportion of 1 : 8, the atomic weight of oxygen cannot be less than 16. No compounds have been found in which a smaller weight of oxygen, relative to one atom of hydrogen, than is represented by the number 16 (approximately), is known to take part in a chemical change. The compound marsh gas contains hydrogen and carbon in the proportion by weight of 1:3. By acting on this compound with chlorine, it is possible to remove the hydrogen from it in four separate portions. By the first action of chlorine upon 16 grammes of marsh gas, I gramme of hydrogen is removed in combination with 35.5 grammes of chlorine, and a compound containing carbon, hydrogen, and chlorine, in the ratio 12:3: 35.5, is formed. By the successive action of chlorine, three other moieties of hydrogen can be thus withdrawn, each being in combination with its equivalent (35.5 parts) of chlorine. The second and third compounds that are formed contain carbon, hydrogen, and chlorine in the ratios 122: (35.5 × 2) and 12:1: (35.5 × 3). The compound produced by the fourth action of chlorine, which withdraws the fourth portion of hydrogen, contains only carbon and chlorine, in the ratio 12:(35.5 × 4). From the fact that the hydrogen contained in marsh gas can thus be removed in four separate portions, the molecule must contain four hydrogen atoms, and therefore the atomic weight of carbon must be at least 12. No compounds of carbon are known in which a smaller weight of carbon, relative to one atom of hydrogen, than is represented by the number 12, takes part in a chemical change. The definition of atomic weight, furnished by considerations of a chemical nature, may be thus stated: the atomic weight of an element, is the number which represents how many times heavier the smallest mass of that element capable of taking part in a chemical change is, than the smallest weight of hydrogen which can so function. The choice of hydrogen as the unit of atomic weights is a purely arbitrary selection; but since atomic weight values can only be determined relatively, it becomes necessary to select some one element and to assign to its atom some particular number to serve as a standard. As hydrogen is the lightest of all elements, Dalton originally adopted it, and arbitrarily fixed unity as the number which should stand for its atomic weight. The disadvantages of this particular unit are twofold: in the first place the number of elements that form hydrogen compounds that are suitable for atomic weight determinations is very small, whereas nearly all the elements form convenient oxygen compounds, or compounds with elements whose atomic weights with reference to oxygen are accurately known, and in actual practice such compounds are almost always made use of for such determinations. In the second place, the exact ratio of the weights of an atom of hydrogen and oxygen is not known with certainty, so that in calculating atomic weights that are determined with reference to oxygen, possible errors may arise. The ratio Hydrogen : Oxygen is not exactly 1 : 16. Various values have been obtained by different experimenters, and at the present time 1: 15.88 is accepted as more nearly the truth. On account of the extreme difficulty of exactly determining this ratio, chemists are now generally agreed in adopting as the unit in all exact determinations of atomic weights a number which is th the weight of the atom of oxygen that is to say, the atomic weight of oxygen is in reality the standard, and is fixed as 16, and the unit, instead of being the weight of one atom of hydrogen, isth of this number. The effect of this change is only of importance in cases of chemical investigation where a high degree of exactitude is required; for purposes of ordinary analyses and chemical calculations the difference that it makes is practically nil Fixing the atomic weight of oxygen at 16 merely raises the atomic weight of hydrogen from 1 to 1.008. As the use of small decimal fractions introduces unnecessary complications which tend to obscure simple processes of reasoning. the approximate atomic weights given in the third column of page 22 will be employed for the most part in the following Introductory chapters. The student will frequently meet with slight discrepancies between the numbers given as the atomic weights of various elements by different writers. Such discrepancies are often due to the fact that in some cases H = 1 is used as the standard, and in others O= 16. For example, the atomic weight of gold will be 195.7 in the first case, and 197.2 in the second; while with the lighter metal aluminium the numbers will be 26.9 as against 27.1. The discrepancy may also arise from the fact that the determination of atomic weights by different experimenters often vary very considerably. With a view to arrive at some uniformity, a conference of representative chemists was held to consider the subject, and the atomic weights finally decided upon by them were published under the title of Internationl Atomic Weights. A revised list of these weights is published annually in the Berichte, and in the fourth column of the table on p. 22 will be found the latest values (1902). 2. Determination of Atomic Weights from Considerations based upon Volumetric Relations. The Law of Gaseous Volumes. In the year 1805 the fact was discovered by GayLussac and Humboldt, that when I litre of oxygen combines with 2 litres of hydrogen the vapour of water (or steam) which was produced occupied 2 litres, the volumes in all cases being measured under the same conditions of temperature and pressure.* This fact led to the discovery of the simple relation existing between the volumes of other reacting gases and the volume of the products: thus it was found that I vol. of hydrogen unites with I vol. of chlorine, and gives 2 vols. of hydrochloric acid. I vol. of hydrogen unites with 1 vol. of bromine vapour, and gives 2 vols. of hydrobromic acid. 2 vols. of hydrogen unite with 1 vol. of oxygen, and give 2 vols. of steam. 2 vols. of carbon monoxide unite with 1 vol. of oxygen, and give 2 vols. of carbon dioxide. I vol. of carbon monoxide unites with 1 vol. of chlorine, and gives 1 vol. of phosgene gas. In the same way with compounds that cannot be obtained by the direct union of their constituent elements, it is found that on being subjected to processes of decomposition similar simple. volumetric relations exist: thus by suitable methods of decomposition 2 vols. of ammonia gas yield 1 vol. of nitrogen and 3 vols. of hydrogen. 2 vols. of nitrous oxide yield 2 vols. of nitrogen and 1 vol. of oxygen. 2 vols. of nitric oxide yield 1 vol. of nitrogen and 1 vol. of oxygen. I vol. of marsh gas yields 2 vols. of hydrogen and some solid carbon, which cannot be evaporated, and therefore its vapour volume is unknown. I vol. of ethylene yields 2 vols. of hydrogen and solid carbon as in the preceding. The observations of these and similar facts gave rise to the law of Gay-Lussac, and it will be seen that there is evidently a close connection between the simple volumetric relations and those existing between the multiple proportions by weight, in which one For the relations of gaseous volumes to temperature and pressure the student is referred to chapter ix., on the general properties of gases. |