ATOMIC THEORY, in chemistry, the theory whose fundamental doctrine is that all matter is composed of ultramicroscopic par ticles, separated by spaces either entirely des titute of matter, or at least containing it in a very attenuated state. Such a theory was taught by Lucretius and other ancient philoso phers, but the views of these early writers were necessarily vague and indefinite, and the atomic theory, as held by chemists of to-day, is gen erally attributed to the English scientist, John Dalton. In the early part of the 19th century Dalton called attention to the fact that when substances combined chemically they do so in certain definite proportions. His reasoning was something like this: In 100 pounds of carbon monoxide there are 42.9 pounds of carbon, and 57.1 pounds of oxygen. In the same weight of carbon dioxide there are 27.3 pounds of carbon, and 72.7 pounds of oxygen. These are merely experimental facts, obtainable by direct analy sis, and they involve no hypothesis whatever. No particular relations are discernable among the numbers stated above; but Dalton dis covered that if the same facts are stated in a different way, a very remarkable relation ap pears. Thus, suppose we calculate what weight of oxygen is combined with each pound of car bon in the two gases. In carbon monoxide we find that there are 57.1 ÷ 42.9=1.33 pounds of oxygen to each pound of carbon, and in carbon dioxide we find that there are 72.7 --"r. 27.3 =2.66 pounds of oxygen to each pound of carbon. One of these numbers being exactly twice the other, we conclude that carbon can unite with oxygen in two proportions, the quantity of oxy gen per unit weight of carbon being twice as great in one case as in the other. Dalton ob served similar relations among other com pounds,— in fact, his theory first occurred to him while he was studying the simpler com pounds of carbon and hydrogen; and after turning the matter over in his mind he came to the conclusion that the facts can best be ex plained by assuming that matter consists of exceedingly minute, indivisible particles or atoms, each of which has a definite weight. When two bodies combine chemically, in con ceived their atoms to come together n pairs, or in threes, or fours, according to the com pound formed; and he devised symbols to rep resent the various elementary bodies and their compounds. Thus oxygen was represented by a circle with a white centre, hydrogen by a circle with a dot in the centre, nitrogen by a circle crossed by a vertical straight line, and carbon by a solid black circle. His notation has no advantages over the one now in com mon use, and hence, in what follows, we shall adopt the modern symbols. As water was the only compound of oxygen and hydrogen that Dalton knew, he naturally represented it by the symbol OH, considering that in it the particles of oxygen and hydrogen are united in pairs. Taking the weight of the hydrogen atom as unity, it follows that the weight of the oxygen atom must be 8, if Dalton's view of the compo sition of water is correct ; for experiment shows that in a given mass of water there is eight times as much oxygen, by weight, as there is hydrogen. Carbon monoxide was rep resented by the symbol OC, and since for each unit of its oxygen (by weight) this gas con tains three-fourths of a unit of carbon, it fol lows that the atomic weight of carbon is three-fourths of that of oxygen. Hence the weight of the carbon atom is 6, the weight of the hydrogen atom being arbitrarily taken, as before, as 1. Carbon dioxide was rep resented by the symbol OCO. Ammonia gas, being the only compound of hydrogen and nitrogen known to Dalton, was represented by the simple symbol NH; and since experi ment shows that ammonia gas contains (by weight) 4% times as much nitrogen as hydro gen, the atomic weight of nitrogen must be 4%, or 4.67. In presenting the foregoing sketch of Dalton's views, use has been made of better experimental data than were available in his time, in order that the relation of his system of atomic weights to the modern system may be more clearly seen. A few of his actual determinations of atomic weights, from the imperfect data that he had, are given in the accompanying table. These were published in Element Atomic Weight Hydrogen 1.0 Nitrogen 4.2 Carbon 4.3 Phosphorus 7.2 Oxygen 5.5 1805, and his general theory of chemical com pounds was given in the first volume of his 'New System of Chemical Philosophy,' pub lished in 1808. All subsequent researches have tended to confirm Dalton's fundamental con ception, that matter is discontinuous in its ulti mate nature, and consists of discrete atoms which come together in definite groups when chemical combination occurs. And we agree with him, to-day, in the belief that the so-called weights' of substances are really the true relative weights of their atoms; the weight of the hydrogen atom being taken as unity. Soon after Dalton's theory had been announced, it was observed that there are simple volumetric relations among gases when they combine.
Thus it was noticed that two volumes of hydro gen combine with one volume of oxygen to form water; that one volume of hydrogen combines with one volume of chlorine to form two vol umes of hydrochloric acid gas; and so on. This being the fact, it was suggested by Avogadro in 1811, and independently by Ampere in 1813, that all gases, when under the same conditions of temperature and pressure, contain the same number of constituent particles per unit of volume. This • principle, known as Law," has been of the great est service to chemistry. Its truth was long questioned, but as it has led to re sults of great value, and has been found to be in conformity, with all other known facts of chemistry, it is now accepted without reserve as a fundamental principle of nature. More over, the kinetic theory of gases has shown that it is a mathematical necessity, if gases are admitted to consist of elastic particles, flying about through space, and colliding with one another, and with the walls of their containing vessels. (See GASES, KINETIC THEORY OF). But if Avogadro's law be admitted to be a fact of nature, it becomes at once, to make an important modification in Dalton's theory. For it is plain that if one cubic inch of hydrogen, containing (say) n atoms, combines with one cubic inch of chlorine, also containing n atoms, to produce two cubic inches of hydro chloric acid gas, containing n constituent parti cles altogether, then the number of such particles in each cubic inch of the hydrochloric acid gas is only n 2 ; whereas Avogadro's law requires us to admit the existence of n particles per cubic inch, in the compound gas as well as in each of its constituents. It follows, therefore, that when the H and the Cl combine, their ulti mate particles do not simply unite in pairs. There is no way to explain the observed facts, consistently with Avogadro's law, unless we as sume that the ultimate particles of H and CI are both compound, and that when these gases com bine, their particles split in two, half a particle of the one combining with half a particle of the other, to produce a whole particle of HCL In other words, Avogadro's law compels us to admit that the little corpuscles of which mat ter is composed, and which we have heretofore called atoms, are really (in some cases, at least) systems composed of still smaller bodies. To distinguish between the two kinds of parti cles— namely, between the systems and their component bodies — it therefore becomes neces sary to introduce a new term. The systems are called "molecules* (literally, "tiny masses"), and their constituent parts are still called "atoms.' To put the case in another way, the smallest parts into which a given substance can be conceived to be divided, without changing its chemical character, are called "molecules'; while "atom* is reserved for the smallest por tion of a substance that can enter into a chemical combination. A molecule is a system of atoms capable of independent existence; and an atom is one of the parts into which the molecule of a substance divides, as a prelimi nary to entering into a chemical combination.
We have but little information, up to the present time, concerning the number of atoms entering into the molecules of the different ele ments. The molecules of cadmium, mercury, zinc and indium are believed to be monatomic, at least when those bodies are in the gaseous state ; so that in these cases there is no differ ence between a molecule and an atom. The same is apparently true of argon, helium, neon, krypton and xenon. Hydrogen, nitrogen, oxygen, selenium and tellurium are believed to be diatomic ; that is, their molecules are believed to contain two atoms each. Phos phorus and arsenic are believed to be tetratomic, their molecules containing four atoms each. Chlorine, bromine and iodine are diatomic at temperatures below 1100° F., but above 1100° their molecules are believed (by some authorities) to break up into single atoms, so that at about 2200° F. two-thirds of the little particles present in these substances are free atoms, while the remaining one-third continue to exist as diatomic molecules. Sul phur is hexatomic at 900° F., but its molecules break up somewhat at higher temperatures, and are practically all diatomic above 1500°. Ac cording to this view of the case, if H stands for the atom of hydrogen and Cl for the atom of chlorine, what happens when a molecule of one of these gases combines with a molecule of the other is not simply H C1=-HCI, because the molecule of hydrogen must he represented by H, and that of chlorine by Cl,. Hence the process of combination consists of two parts, the first of which is H, Cl, = H + H + CI + CI Molecule Molecule Atoms of Atoms of of hydrogen of chlorine hydrogen chlorine The atoms of hydrogen and chlorine, thus set free, then combine to form hydrochloric acid.
It is then written as follows: