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JOURNAL OP CAEMIDU.EDUCATION
NOVBMBB~. 1926
THE STRUCTURE OF MATTER: A BRIEF REVIEW OF PRESENTDAY CONCEPTIONS. 11. THE ATOM AND RADIATION MAURICE L. HUGGINS, S T ~ UNO IVER~SITY, CALIPORNTA This article deals with light radiation and its emission and absorption by atoms and molecules. From studies of such radiation a large part of our present knowledge of the atom has been derived. The Quantum Theory Present-day theories of the interaction between atoms and radiation are all based largely on the quantuw theory. This was first proposed by Planck in 1900 in order to explain the distribution among different wavelengths of the radiation absorbed or emitted by a black body. Planck's fundamental assumption was that radiant energy is absorbed (or, in a later form of the theory, emitted) only in discrete bundles or quanta, each containing an amount of energy equal to Planck's constant, h, times the frequency, v, of the radiation. (The frequency, v , is inversely proportional to the wave-length, X.) The Bohr Theory of the Hydrogen Atom Bohr, in 1913, applying Planck's quantum theory to Rutherford's picture of the atom as a miniature solar system, proposed the theory which has sewed as a foundation for practically all subsequent work in this field. Considering the neutral hydrogen atom, consisting of one proton and one electron, he postulated: (1) That the electron is constantly rotating in a circular orbit around the nucleus, ordinarily w'tkout radiating any energy. (According to classical electromagnetic theory, such a rotating electrical charge would continuously emit radiation and, losing energy in this way, spiral into the nucleus.) (2) That only cerlain orbits are stabl+those in which the angular momentum is equal to an integral multiple of Planck's constant, h, divided by 2s. (3) That when an electron changes its orbit, radiation is absorbed or emitted having a frequency, v, determined by the equation: AE = hr
where A E is the difference between the energy of the atom in its final and initial states. (4) That when the electron is i n one of its stable orbits, Coulomb's law, giving the force between it and the nucleus, holds as well as the usual centrifugal force law. Starting with these hypotheses, Bohr was able to calculate quantitatively, without the use of any arbitrary constants, the frequencies (and so
the wave-lengths) of aU the lines in the normal hydrogen spectrum. Applying the same assumptions to the elucidation of the spectra obtained from the singly ionized helium atom, consisting of nucleus and one electron, quantitative agreement was again obtained. By further assuming elliptical orbits as well as circular, Sommerfeld and others have shown that the theory very satisfactorilyaccounts for thefine structures of the spectrum l i n e s m a n y of the lines which early observations showed as single lines actually consisting of two or more lines very close together. Moreover, although the application of the theory to more complex atoms, to molecules, to the effect of strong electrical and magnetic fields on spectrum lines, and to other problems has been much more difficult, especially from a mathematical standpoint, the theory has met with success after success in (qualitatively and semi-quantitatively) accounting for experimental results. This makes it seem almost certain that the mathematical equations developed from the theory are fundamentally correct, and no one has yet offered another picture of the atom which will give these equations. One of the most recent additions to the Bohr theory, but one which has already received considerable verification, is the idea that the electron, while in rotation about the atomic nucleus, is a t the same time spinning, with constant angular momentum, about an axis of its own, much as the earth spins about its axis while rotating about the sun. The Bohr Theory Applied to More Complex Atoms The application of the Bohr theory to more complex atoms than hydrogen and ionized helium, although not productive of such precise quantitative checks with experiment, largely because of the mathematical difficulties involved in dealmg with a problem of three or more bodies, has enabled physicists to progress very far in the interpretation of atomic spectra and in the study of the distribution of electrons among various "energy levels" in their atoms. Their assignments of electrons to energy levels conform very closely to the assignment to "shells" given in the first paper of this series; in general the spectroscopic evidence is in good agreement with the chemical evidence. Some of the shells, i t is true, are further subdivided by physicists, but such subdivisions do not seem to be of much importance chemically. According to the physicists' picture of these complex atoms the electrons are all in constant orbital rotation about the nucleus.' Whether or not this picture will stand the test of time remains to be seen. On the whole, it has so far proved very satisfactory. Some of the most recent work using the spinning electron theory places same electrons in "orbits" having zero angular momentum. If this is correct, it would seem that these "orbits" must be mere oscillations.
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SOVBMBER, 1926
The Bohr Theory Applied to Molecules The spectrum of light coming from uncombined atoms consists of a few "lines," each produced by light of definite frequency, superimposed on a background of "general" or "white" radiation. From molecules "band spectra" are obtained, each band consisting of a great many lines, spaced in some regular manner and usually close together. The interpretation of band spectra has up to the present been limited almost entirely to simple diatomic molecules. In interpreting these spectra i t is assumed that not only are the motions of the electrons "quantized," but so are the vibrations of the atoms relative to each other and also the rotations of the molecule as a whole. In other words, the molecule can have only certain definite amounts of vibrational energy, of rotational energy, and of kinetic and potential energy due to the electrons in their orbits. When any one or two or all of these change, light is emitted or absorbed, its frequency being determined by the total change in energy: AE = hv
The spectrum lines in a givm band are all said to be due to molecules undergoing the same electronic change (from one energy level to another) and the same change of vibrational energy, but different changes of rotational energy. Different bands in a "band system" result from different changes in vibrational energy; different "band systems" in a "systemseries" have their origin in different electron "jumps." From such spectra it is sometimes possible to determine the moment of inertia of the molecule (and so the distance apart of the atoms), the magnitude of the force holding the atoms together, etc. Those working in this field will probably be able to teach the chemist much in regard to molecular structure and inter-atomic forces in the next few years. The Nature of Light It is evident from the foregoing that the problem of the structure of the atom is very closely connected with that of the nature of light. As regards the latter, the present-day physicist finds himself in an apparently hopeless dilemma, and no one knows what will be the way out. The phenomena of light interference seem absolutely to require light waves for their explanation. To take a simple case, light from a source S (Fig. I), passing through a single slit A, will affect a photographic plate a t C; but if there is a similar slit B so placed that the diierence between the paths SAC and SBC is one-half wave-length, there will be no effect on the plate a t C. Using the wave theory of light the explanation is simple, but it is difficult to see how any other theory, such as one assuming light particles, could account for this phenomenon, for how can the effect of
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one particle on a photographic plate be nulliiied by another particle amving later? On the other hand, as has already been mentioned, the distribution of frequencies in the radiation absorbed or emitted by a black body seems to require that radiant energy be either absorbed or emitted by an atom in quanta.
I
Other very definite evidence is furnished by the photo-electric effect. If light of a definite frequency is allowed to fall upon a metallic surface, electrons are emitted from that surface, provided the frequency of the light is greater than a certain minimum value, characteristic of the metal, but none a t all provided the frequency is less than this minimum. This fact and the relationships experimentally found , connecting the frequency and SL L -- - -c intensity of the incident light with the number and velocities of the emitted electrons seem inexplicable on the basis of a FIG.1. wave theory, but they are in good agreement with the "light dart" theory proposed by Einstein as a further development of Planck's quantum theory. Another phenomenon which seems to be impossible of explanation from the standpoint of the wave theory but is in quantitative agreement with Einstein's extreme form of the quantum theory is the Compton effect. When light of a frequency v is allowed to fall upon matter, some of the light scattered has a lower frequency (longer wave-length) than the original. Such are the problems that physicists are now trying to solve. They are mentioned here chiefly because of the close relationship they bear to the structure of the atom and the molecule and to the mechanism of chemical reactions. Developments of great importance to chemists are sure to come, the writer believeprobably in the very near f u t u r e f r o m studies and experiments in this field.
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The Radiation Theory of the Mechanism of Chemical Reaction
A few words might appropriately be added here concerning the radiation theory of chemical reaction. It is well known that light radiation is effective in catalyzing many chemical reactions. Some have supposed that light (not necessarily visible light, of course) of some particular frequency, or perhaps light having a frequency greater than a certain minimum, is necessary before any reaction will go. Whether or not this is the case no one can yet say, but the experimental evidence seems to be quite definitely against the simple theory that for a given reaction only a single frequency.
determined by putting the energy change in the reaction equal to hv, is effective. Enalum.-Through inadvertence in proof-reading, an error crept into the first article of this series. On page 1111 the mass of the electron is given as 8.999 X 10-". The correct value is 8.999 X lo-'% We acknowledge the kindness of H. Louis Jackson of R. I. State College in calling our attention to this error.
