Atomic Theory

(Chem.) Every body is supposed to consist of atoms of unknown size, form, and weight, which, being infinitely hard, cannot be further subdivided. The atoms of elements are simple, those of compounds are compound. No atom has ever been seen, even by the most powerful microscopes, although particles of bodies le than 싸이이이이이 of an inch in diameter have been discerned by their aid; the forms of atoms are, therefore, unknown. Although the actual size and weight of the ultimate atoms of bodies cannot be ascertained, it is easy to discover their relative size and weight. For instance, it is found by experiment that 1 grain of hydrogen combines with 80 grains of bromine to form a definite compound, and never in any other proportion; it is therefore supposed that the number of atoms of each body is the same, but that the weight of each atom is as 1 to 80. This is borne out by the fact that bromine is exactly 80 times as heavy as hydrogen, a cubic inch of the one uniting with exactly 1 cubic inch of the other. The numbers 1 and 80, therefore, represent the weight of the atoms or the atomic weights of these bodies; and it follows that 81 is the atomic weight of the compound formed by their union. These numbers are also termed the chemical equivalent or combining proportion of these substances. All the other elements have their combining proportions or atomic weights, and are subject to the same laws as hydrogen and bromine. It often happens that double, treble, and quadruple atoms of one element will unite with a single atom of another element to form a compound; for instance, the following compounds of nitrogen and oxygen occur:Nitrogen, 14 grains; Oxygen, 8 grains. IMG:content-0389.png:[ocr errors][ocr errors][ocr errors] 14 14 " 14 14 " IMG:content-0390.png:[ocr errors][ocr errors] " IMG:content-0391.png:[ocr errors] 16 24 " 32 " " 40 66 Here we find one atom of nitrogen uniting with a single, double, treble, quadruple, and quintuple atom of oxygen, and never in any other proportion -as N15 to 08, or N14 to 07, for example. This is called the doctrine of definite proportions , and is a consequence of the theory of atoms of a constant size and weight laid down above. It was first put forth by John Dalton, of Manchester. It has been said above, that the atoms of the same body are of constant size; but it happens in several cases that the atoms of different bodies are of different sizes. Thus, though it is found that one cubic inch of hydrogen unites with one cubic inch of bromine, it by no means follows that equal volumes of carbon and iron unite in chemical combination. There is great diversity in the relative sizes of elementary atoms, those of carbon being only half the size of those of iron. The weight of an atom of carbon would be 12, and that of an atom of iron would be 56; but the quantity of carbon would only take up half the space of the quantity of iron. It has been found that bodies of the same com bining volume agree strongly in their properties. Iodine, chlorine, and bromine are very similar in their reactions; and there are several other groups of elements formed on the same basis; (see ATOMIC VOLUME.) To sum up, atomic weight is the relative weight of the atoms of bodies, or, which is the same thing, the proportion in which they unite. Rensoning on this theory, tables have been constructed of the atomic weights of all the elements; the atomic weights of compounds being evidently the sum of the atomic weights of their constituents. Hydrogen has been taken as the unitary body by certain chemists, from being the lightest of all known substances. Others have taken 100 parts of oxygen as the standard. This latter arrangement, by increasing the figures, renders it le convenient to work. Atomic Weights, Combining Proportions, or Chemical Equivalents of the Elements. IMG:content-0392.png:[blocks in formation] These atomic weights have not been decided upon without great difficulty. They have been fixed by various kinds of evidence, obtained by very numerous and often greatly varied experiments. The result has been to make important changes in the tables of atomic weights formerly given; many of the weights now accepted being double those formerly used, while some are three and others four times the old numbers. Many efforts have been made to deduce some regular relation between the numbers and the properties of elementary atoms, the most succe ful of these being that made by Mendeleeff, who formulated what is known as the Periodic Law, which states that the properties of an element are a function of its atomic weight. He, and others before him, found that when the elements are arranged in progre ive order of their atomic weights a certain regularity is observable in the succe ion of elements with analogous chemical properties. So well marked is this, that tables of the elements can be formed based on their double relation of numbers and properties, and in these tables certain gaps appear which it was predicted would yet be filled by elements of certain given weights and properties. It is interesting to be able to state that these predictions have been fulfilled in the discovery and character of certain elements new to science, notably Gallium and Germanium. This adds greatly to the standing of the theory. Atomic Volume is the relative size of the combining proportions or atoms of bodies, just as atomie weight is the relative weight of their combining proportions or atoms. It is determined by dividing the atomic weight by the specific gravity. As the principal researches on atomic weights have been made in Germany, the oxygen standard is generally used in calculating them. As before stated (see ATOMIC THEORY), substances fall into several well-marked groups, po e ing analogous properties, and giving isomorphous compounds. The following table will be sufficient to show this fact: IMG:content-0393.png:[blocks in formation] IMG:content-0394.png:[blocks in formation] IMG:content-0395.png:[blocks in formation] 27 IMG:content-0396.png:[blocks in formation] Antimony ( Stibium ).........Sb 120 IMG:content-0397.png:[blocks in formation] Arsenic IMG:content-0398.png:[blocks in formation] IMG:content-0399.png:[blocks in formation] IMG:content-0400.png:[blocks in formation] Silver.. Beryllium IMG:content-0401.png:[blocks in formation] IMG:content-0402.png:[blocks in formation] IMG:content-0403.png:[blocks in formation] Bismuth. IMG:content-0404.png:[blocks in formation] Boron.. IMG:content-0405.png:[blocks in formation] IMG:content-0406.png:[blocks in formation] Bromine. IMG:content-0407.png:[blocks in formation] IMG:content-0408.png:[blocks in formation] Iodine. 1587 ..320 4-95 IMG:content-0409.png:[blocks in formation]