Atoms and Molecules

230 NATURE [Nov~1rnER 6, 1919 electricity. In the model atom proposed by Sir of Sommerfeld, Epstein, and others. The general J. J. Thomson the electrons were supposed to be ised theory has proved very fruitful in accounting embedded in a sphere of positive electricity of in a formal way for many of the finer details of about the dimension of the atom as ordinarily spectra, notably the doubling of the lines in the understood. Experiments on the scattering hydrogen spectrum and the explanation of the of a-particles through large angles as the complex details of the Stark and Zeeman effects. result of a single collisioh with a heavy In these theories of Bohr and his followers it is atom showed that this type of atom was not cap assumed that the electrons are in periodic orbital able of accounting for the facts unless the positive motion round the nucleus, and that radiation only sphere was much concentrated. This led to the arises when the orbit of the electron is disturbed nucleus atom of Rutherford, where the positive in a certain way. Recently Langmuir, from a charge and also the mass of the atom are supposed consideration of the general physical and chemical to be concentrated on a nucleus of minute dimen properties of the elements, has devised types of sions. The nucleus is surrounded at a distance by atom in which the electrons are more or less fixed a distribution of negative electrons to make it in position relatively to the nucleus like the atoms electrically neutral. The distribution of the ex of matter in a crystal. It appears necessary, in ternal electrons on which the ordinary physical Langmuir's theory, to suppose that electrons, 10 and chemical properties of the atom depend is addition to their electrical charges, are endowed almost entirely governed by the magnitude of the with the properties of a mag·netic doublet, so that positive charge. The experiments by Marsden at a certain distance the forces of attraction and and Geiger on the scattering of the a-particles, repulsion between two electrons counterbalance and also on the scattering of X-rays by Barkla, one another. show that the resultant units of charge on the The whole question of the possible arrange nucleus of an element is about equal to its ments and motion of the external electrons in an atomic number when arranged in order of increas atom or molecule still remains a matter of much ing atomic weight. Strong proof of the correct doubt and speculation. While there are strong ness of this point of view has been given by the indications that the conception of the nucleus atom work of Moseley on the X-ray spectra of the is in the main correct, we are still very uncertain elements, for he has shown that the properties of of the laws controlling the position of the external an element are defined by a whole number which electrons on which the ordinary physical and changes by unity in passing from one element to chemical properties depend. The study of the the next. It is believed that the lightest element, light spectra and also of the X-ray spectra already hydrogen, has a nuclear charge of one, helium promise to throw new light on this very difficult of two, lithium of three, up to the heaviest but fundamental problem. element, uranium, of charge 92. From the above hurried survey of the progress Radioactive evidence indicates that the nucleus of atomic physics, it wiil be seen that the investi contains both positively charged masses and nega gations of the past twenty-five years have dealt tive electrons, the positive charge being in excess. mainly with three great outstanding problems, Apart from the difficulty on the ordinary laws of viz., the nature of electricity, the structure of the electric forces of explaining why the nucleus holds atom, and the nature of radiation. While great together, there is a fundamental difficulty of ac additions have been made to our knowledge of counting for the stability of the external electrons these questions leading to a much wid~r outlook, on the ordinary laws of dynamics. To overcome ' we cannot but recognise that much still remains to this difficulty, Bohr has applied the quantum be done before we are certain that we are building theory to define the position of the electrons and on a firm foundation for the future. Notwith to account for the spectra of the lighter atoms standing the prolonged halt during the war, the and has made suggestions of the structure of the scientific outlook is one of good augury for the simpler atoms and molecules. Space does not immediate future, and there is every prospect that allow me to discuss the important developments the vigorous attack on these outstanding problems that have followed from Bohr's theory by the work will be continued. ATOMS AND MOLECULES. BY PROF. FREDERICK SODDY, F.R.S. { T may be doubted whether, fifty years ago, so wedded to pure hypotheses and unverifiable chemists and physicists believed very deeply assumptions, then, apparently, for ever beyond the in the actual reality of the molecules and atoms, power of being actually apprehended and which they used as convenient and simplifying demonstrated. That the modern student of conceptions to interpret the behaviour of matter. physical science believes in the reality of the The half-century, indeed, has not passed without existence of his atoms and molecules, as much strong protest from the thermodynamical school as he does in that of chairs, tables, and lamp· of physical chemistry that the science should be posts, probably sufficiently epitomises one of the NO. 2610,VOL. 104] ©1919 Nature Publishing Group NovEMBER 6, r9r9] NATURE 231 most striking features of the change of outlook exists in a form capable of free independent move since NATURE made its first appearance in 186g. ment. Vague ideas of their actual individual mass, size, Cleared of these ambiguities, the conception of shape, and constitution have been or are being the. individual molecule has become very real. replaced more and more by exact quantitative We have been led by Perrin, and the mathe knowledge, which invites our literal acceptance matical physicists who paved the way for his and grows in fruitfulness the more implicitly it experimental work, to recognise the Brownian is used as the basis for further investigations. movement as but one aspect of foe perpetual But the latter half of the period under review motion of the molecules, which, though invisible witnessed an even greater change of outlook. to the naked eye, becomes swift and ceaseless for The atom, since the discovery of radio-activity in particles even of the scale of minuteness resolved 1896, has ceased to be the smallest coin of the by the microscope, and we can extrapolate with realm of material change. The farthings of 1869 assurance to the minuter world which science had have proved to resemble 100oz. notes, and the long before visualised by faith. potentialities of the world in consequence have Or, again, we may follow Langmuir, with none been multiplied a million times. The change of of the feeling of hesitancy and diffidence that the single atom of matter is well within the range would have held back an earlier generation, into of direct perception by the senses, and, stranger the explanation of catalysis, adsorption, and still, the change reveals that, under the image allied phenomena, as caused by surface layers of and superscription of the same Cresar, coins of molecules "one molecule thick." Nor do we con different mass and mintage have been circulating sider it fanciful to explain the spreading of animal unsuspected in the chemist's currency. and vegetable oils upon water and the non-spread As regards the physical reality of molecules, ing of mineral oils, as due to the attempt, in the by no means the least important factor contribut first case, of the one end, the soluble glycerine ing to the result has been the recognition that, if ester end, of the rod-like molecule to dissolve in the molecules were not the smallest parts of the water, and the refusal of the other end, the matter capable of free independent existence and insoluble, hydro-carbon, or oily end, to do so. motion, heat would not be the final permanent Wherefore the molecules of such oils stand up form which all kinetic energy liberated in the on end and cover the surface with a one-molecule world assumes. The limit that fixes the physical thick layer of the oily ends of the molecules, sub-division of matter limits also the sub-division whereas the mineral oils, with molecules oily at of motion. Though in the real world of matter both ends, do not spread ! Real in one sense as in bulk, as contrasted with the ideal fictions of the structural formulre of organic compounds have mathematics, friction and imperfect elasticity been for many decades, an earlier generation quickly reduce all moving masses to apparent rest, would scarcely have thought of this. that "rest " is the perpetual heat motion of the The discovery that the X-rays are of a char molecules, which, literally and necessarily, must acter identical with light, but of wave-length of be perfectly frictionless and elastic because they the order of one ten-thousandth of that of light are the smallest particles capable of free inde of the visible spectrum, has made the structure pendent motion, and no smaller particles exist of crystalline solids as open tq direct examination among which their motion can be further dis as the ten-thousand-fold coarser structure of the tributed.

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