Atomic weight

“ We have seen that the view, so strongly supported by recent experiments, that the atoms of the elements are aggregations of simpler systems, involves the admission that losses or gains of mass or weight must occur in the formation of the heavier atoms. But we know that the ratio of mass to weight is the same for all substances, from hydrogen, the lightest, up to uranium, the heaviest, and even, as Southern's experiments on uranium and my own on radium have shown, for radio-active substances. ”

“ All nuclei (except hydrogen) weigh less than the sum of the weights of the particles in them.
For example, the atomic weight of a proton is 1.00812 and that of a neutron is 1.00893. (These are relative weights based on an internationally accepted scale.) It would seem then that a nucleus of helium containing two protons and two neutrons should have an atomic weight of 2 × 1.00812 plus 2 × 1.00893 or 4.0341. Actually the atomic weight of helium as measured by the mass spectrograph is only 4.0039. ”

“ P. 13.—The abbreviations used in the atomic symbols are taken from the Latin or Greek names, and when these differ from the English there seems to be no correspondence between the name of the element and its atomic symbol; as Au for gold.
Hydrogen is the lightest form of matter known, and the weight of its atom is taken as the unit of the system of weights. In the table the numbers in the column of atomic weights give the weight of one atom of each substance as compared with one atom of hydrogen. For instance, an atom of aluminum is twenty-seven times as heavy as an atom of hydrogen. ”

“ The atomic weight of hydrogen, 1·008, is confirmed. The elements helium, carbon, nitrogen, oxygen, fluorine, phosphorus, sulphur, arsenic, iodine and sodium are found to be simple bodies with whole-number atomic weights. On the other hand, boron, neon, silicon, chlorine, bromine, krypton, xenon, [105] mercury, lithium, potassium and rubidium are found to be mixtures. What is specially of interest is that the indicated atomic mass of each of the constituents is a whole number. ”

“ As soon as an atom is formed out of protyle it is a store of energy, potential (from its tendency to coalesce with other atoms by gravitation or chemically) and kinetic (from its internal motions) . To obtain this energy, the neighbouring protyle must be refrigerated by it, and thereby the subsequent formation of other atoms will be accelerated. But with atomic matter the various forms of energy which require matter to render them evident begin to act; and, amongst others, that form of energy which has for one of its factors what we now call atomic weight. ”

“ Now if we constructed models of the atoms out of different materials, the atomic weight would be but one factor out of many which would influence the physical and chemical properties of the model, we should require to know more than the atomic weight to fix its behaviour. If we were to plot a curve representing the variation of some property of the substance with the atomic weight we should not expect the curve to be a smooth one, for instance two atoms might have the same atomic weight and yet if they were made of different materials have no other property in common. ”

“ Thus the atomic weight of uranium is 240, that of thorium 233, that of radium 225 or, according to a recent spectroscopic test, by Runge, 256. There is no other property in which these three substances are at all alike. In their chemical characteristics they are extremely different. Now, according to our modern mechanical theory of heat, the atoms of all substances are in extremely rapid rotation. ”

“ When the missing atomic weight 0.0302 is multiplied by the square of the velocity of light according to Einstein’s theory, it is found to represent a tremendous amount of energy. Indeed, the energy released in forming a helium nucleus from two protons and two neutrons turns out to be seven million times that released when a carbon atom combines with an oxygen molecule to produce a molecule of carbon dioxide in the familiar process of combustion. ”

“ We infer its properties indirectly from the behaviour of the sensible masses with which we can deal more directly. Hence, at most, the statement that the atom of oxygen has a certain weight means no more than that we can for our practical purposes disregard any possible individual divergences from this value. The oxygen atoms, if they really exist, might actually fluctuate in individual atomic weight about an average ”

“ It is easy, for example, to conceive that the heavy metals may lie somewhat deeper down within the Sun or star than those of low atomic weight. In the case of the Sun, there seems a clear connection between atomic weight and the distinctness with which the element is recognized in the spectrum of the photosphere, the lower atomic weights showing themselves more conspicuously. ”

“ This leads us to the view that the electrons in an atom, if they exceed a certain number, are divided up into groups, into a series of spherical layers, like the coatings of an onion, separated from each other by finite distances, the number of such layers depending upon the number of electrons in the atom, and thus upon its atomic weight.
The electrons in the outside layer will be held in their places less firmly than those in the inner layers; they are more mobile, and will arrange themselves more easily under the forces exerted upon them by other atoms. ”

“ A diminution of mass means a liberation of energy proportional to it, so that the amount of energy liberated in the formation of these lighter elements will be proportional to the defect of this atomic weight from the nearest integer. ”

“ The materialists and energists are now on the high-road to reconciliation, and we are permitted to feel that an explanation may soon be forthcoming for the well-known relation between specific heat and atomic weight, and for that existing between spectral phenomena and atomic weight. ”

“ While the factors of kinetic and of gravitational energy, velocity and momentum, on the one hand, and force and distance, on the other, are simply related to each other, the capacity factors of other forms of energy—surface, in the case of surface-energy; volume, in the case of volume-energy; entropy for heat; electric capacity when electric charges are being conveyed by means of ions; atomic weight, when chemical energy is being gained or lost—all these are simply connected with the fundamental chemical capacity, atomic weight or mass. ”

“ It is to be observed that the attribution of different weights to different atoms is an indispensable feature of the atomic theory in chemistry, especially in view of the combination of gases in simple ratios of volume, so as to give rise to gaseous products bearing a simple ratio to the volumes of its constituents, and in view of the law of Ampère and Clausius, according to which all gases, of whatever nature or weight, contain equal numbers of molecules in equal volumes. ”

“ We are not wholly without hope that the [242] real weight of each such atom may some day be known, not merely the relative weight of the several atoms, but the number in a given volume of any material that the form and motion of the parts of each atom, and the distance by which they are separated, may be calculated that the motions by which they produce heat, electricity, and light may be illustrated by exact geometrical diagrams and that the fundamental properties of the intermediate and possibly constituent medium may be arrived at. ”

“ If we remember that our atomic weights are confessedly maximum weights, and not minimum weights—although they are almost certainly also the true atomic weights—such a view would be, at least, worthy of some consideration. ”