230 



NATURE 



[November &, igig 



electricity. In the model atom proposed by Sir 

 J. J. Thomson the electrons were supposed to be 

 embedded in a sphere of positive electricity of 

 about the dimension of the atom as ordinarily 

 understood. Experiments on the scattering 

 of a-particles through large angles as the 

 result of a single collision with a heavy 

 atom showed that this type of atom was not cap- 

 able of accounting for the facts unless the positive 

 sphere was much concentrated. This led to the 

 nucleus atom of Rutherford, where the positive 

 charge and also the mass of the atom are supposed 

 to be concentrated on a nucleus of minute dimen- 

 sions. The nucleus is surrounded at a distance by 

 a distribution of negative electrons to make it 

 electrically neutral. The distribution of the ex- 

 ternal electrons on which the ordinary physical 

 and chemical properties of the atom depend is 

 almost entirely governed by the magnitude of the 

 positive charge. The experiments by IVIarsden 

 and Geiger on the scattering of the o-particles, 

 and also on the scattering of X-rays by Barkla, 

 show that the resultant units of charge on the 

 nucleus of an element is about equal to its 

 atomic number when arranged in order of increas- 

 ing atomic weight. Strong proof of the correct- 

 ness of this point of view has been given by the 

 work of Moseley on the X-ray spectra of the 

 elements, for he has shown that the properties of 

 an element are defined by a whole number which 

 changes by unity in passing from one element to 

 the next. It is believed that the lightest element, 

 hydrogen, has a nuclear charge of one, helium 

 of two, lithium of three, up to the heaviest 

 element, uranium, of charge 92. 



Radioactive evidence indicates that the nucleus 

 contains both positively charged masses and nega- 

 tive electrons, the positive charge being in excess. 

 Apart from the difficultv on the ordinary laws of 

 electric forces of explaining why the nucleus holds 

 together, there is a fundamental difficulty of ac- 

 counting for the stability of the external electrons 

 on the ordinary laws of dynamics. To overcome 

 this difficulty, Bohr has applied the quantum 

 theory to define the position of the electrons and 

 to account for the spectra of the lighter atoms 

 and has made suggestions of the structure of the 

 simpler atoms and molecules. Space does not 

 allow me to discuss the important developments 

 that have followed from Bohr's theorv bv the work 



of Sommerfeld, Epstein, and others. The general- 

 ised theory has proved very fruitful in accounting 

 in a formal way for many of the finer details of 

 spectra, notably the doubling of the lines in the 

 hydrogen spectrum and the explanation of the 

 complex details of the Stark and Zeeman effects. 

 In the.se theories of Bohr and his followers it is 

 assumed that the electrons are in periodic orbital 

 motion round the nucleus, and that radiation only 

 arises when the orbit of the electron is disturbed 

 in a certain way. Recently Langmuir, from a 

 consideration of the general physical and chemical 

 properties of the elements, has devised types of 

 atom in which the electrons are more or less fixed 

 in position relatively to the nucleus like the atoms 

 of matter in a crystal. It appears necessary, in 

 Langmuir 's theory, to suppose that electrons, m 

 addition to their electrical charges, are endowed 

 with the properties of a magnetic doublet, so that 

 at a certain distance the forces of attraction and 

 repulsion between two electrons counterbalance 

 one another. 



The whole question of the possible arrange- 

 ments and motion of the external electrons in an 

 atom or molecule still remains a matter of much 

 doubt and speculation. While there are strong 

 indications that the conception of the nucleus atom 

 is in the main correct, we are still very uncertain 

 of the laws controlling the position of the external 

 electrons on which the ordinary physical and 

 chemical properties depend. The studv of the 

 light spectra and also of the X-ray spectra already 

 promise to throw new light on this verv difficult 

 but fundamental problem. 



From the above hurried survey of the progress 

 of atomic physics, it will be seen that the investi- 

 gations of the past twenty-five years have dealt 

 mainly with three great outstanding problems, 

 viz., the nature of electricity, the structure of the 

 atom, and the nature of radiation. U'hile great 

 additions have been made to our knowledge of 

 these questions leading to a much wid'jr outlook, 

 we cannot but recognise that much still remains to 

 be done before we are certain that we are building 

 on a firm foundation for the future. Notwith- 

 standing the prolonged halt during the war, the 

 scientific outlook is one of good augury for the 

 immediate future, and there is every prospect that 

 the vigorous attack on these outstanding problems 

 will be continued. 



ATOMS AND MOLECULES. 



By Prof. Frederick Soddy, F.R.S. 



f T may be doubted whether, fifty years ago, 

 chemists and physicists believed very deeply 

 in the actual reality of the molecules and atoms, 

 which, they used as convenient and simplifying 

 conceptions to interpret the behaviour of matter. 

 The half-century, indeed, has not passed without 

 strong protest from the thermodynamical school 

 of physical chemistry that the science should be 

 NO. 2610, VOL. 104] 



so wedded to pure hypotheses and unverifiable 

 assumptions, then, apparently, for ever beyond the 

 power of being actually apprehended and 

 demonstrated. That the modern student of 

 physical science believes in the reality of the 

 existence of his atoms and molecules, as much 

 as he does in that of chairs, tables, and lamo- 

 posts, probably sufficiently epitomises one of the 



