414 



Supplement to ''Nature,'' Scph/Jider 15, 1923 



fine structure of the lines and the very complicated 

 changes observed when the radiating atoms are ex- 

 posed in a strong magnetic or electric field. Under 

 ordinary conditions the electron in the hydrogen atom 

 rotates in a circular orbit close to the nucleus, but if 

 the atoms are excited by an electric discharge or other 

 suitable method, the electron may be displaced and 

 occupy any one of the stable positions specified by the 

 theory. In a radiating gas giving the complete 

 hydrogen spectrum there will be present many different 

 kinds of hydrogen atoms, in each of which the electron 

 describes one of the possible orbits specified by the 

 theory. On this view it is seen that the variety of 

 modes of vibration of the hydrogen atom is ascribed, 

 not to complexity of the structure of the atom, but to 

 the variety of stable orbits which an electron may 

 occupy relative to the nucleus. This novel theory of 

 the origin of spectra has been developed so as to apply 

 not to hydrogen alone but to all the elements, and has 

 been instrumental in throwing a flood of light on the 

 relations and origin of their spectra, both X-ray and 

 optical. The information thus gained has been applied 

 by Bohr to determine the distribution of the electrons 

 round the nucleus of any atom. The problem is ob- 

 viously much less complicated for hydrogen than for a 

 heavy atom, where each of the large number of electrons 

 present acts on the other, and where the orbits described 

 are much more intricate than the orbit of the single 

 electron in hydrogen. Notwithstanding the great diffi- 

 culties of such a complicated system of electrons in 

 motion, it has been possible to fix the quantum numbers 

 that characterise the motion of each electron, and to 

 form at any rate a rough idea of the character t)f the 

 orbit. 



These planetary electrons divide themselves up into 

 groups, according as their orbits are characterised by 

 one or more equal quantum numbers. Without going 

 into detail a few examples may be given to illustrate 

 the conclusions which have been reached. As we have 

 seen, the first element, hydrogen, has a nuclear charge of 



1 and I electron ; the second, helium, has a charge 2 and 



2 electrons, moving in coupled orbits on the detailed 

 nature of which there is still some uncertainty. These 

 two electrons form a definite group, known as the K 

 group, which is common to all the elements except 

 hydrogen. For increasing nuclear charge the K group 

 of electrons retains its characteristics, but moves with 

 increasing speed, and approaches closer to the nucleus. 

 As we pass from helium of atomic number 2 to neon, 

 number 10, a new group of electrons is added consisting 

 of two sub-groups, each of four electrons, together 

 called the L group. This L group appears in all atoms 

 of higher atomic number, and, as in the case of the K 

 group, the speed of motion of the electrons mcreases, 

 and the size of their orbits diminishes with the atomic 

 number. When once the L group has been completed 

 a new and still more complicated M group of electrons 

 begins forming outside it, and a similar process goes on 

 until uranium, which has the highest atomic number, is 

 reached. 



It may be of interest to try to visualise the concep- 

 tion of the atom we have so far reached by taking for 

 illustration the heaviest atom, uranium. At the centre 

 of the atom is a minute nucleus surrounded by a 

 swirling group of 92 electrons, all in motion in definite 



1 lie a 

 not 1 



piyj 



orbits, and occupying but by no means filling a volum. 

 very large compared with that of the nucleus. S' : . 

 of the electrons describe nearly circular orbits r* 

 the nucleus ; others, orbits of a more elliptical 

 with axes rotating rapidly round the nucleus, i m,- 

 motion of the electrons in the diflerent groups is not 

 necessarily confined to a definite region of the atom, 

 but the electrons of one group may penetrate deeply 

 into the region mainly occupied by another grou| ~ 

 thus giving a type of inter-connexion or couplir 

 between the various groups. The maximum sp< ' 

 any electron depends on the closeness of the appr 

 to the nucleus, but the outermost electron will have a 

 minimum speed of more than 1000 kilometres per 

 second, while the innermost K electrons have an 

 average speed of more than 150,000 kilometres '" • 

 second, or half the speed of light. When we vLsi 

 the extraordinary complexity of the electronic syMcin 

 we may be surprised that it has been possible to finrl 

 any order in the apparent medley of motions. 



In reaching these conclusions, which we owe largely 

 to Prof. Bohr and his co-workers, ever>' available kind 

 of data about the different atoms has been taken into 

 consideration. A study of the X-ray spectra, in par- 

 ticular, affords information of great value as to the 

 arrangement of the various groups in the atom, whik 

 the optical spectrum and general chemical properties 

 are of great importance in deciding the arrangements 

 of the superficial electrons. While the solution of the 

 grouping of the electrons proposed by Bohr has been 

 assisted by considerations of this kind, it is not empirical 

 in character, but has been largely based on general 

 theoretical considerations of the orbits of electrons that 

 are physically possible on the generalised quantum 

 theory. The real problem involved may ht illustrated 

 in the following way. Suppose the gold nucleus be 

 in some way stripped of its attendant seventy-nine 

 electrons and that the atom is reconstituted by the 

 successive addition of electrons one by one. According 

 to Bohr, the atom will be reorganised in one way only, ™ 

 and one group after another wUl successively form and j| 

 be filled up in the manner outlined. The nucleus atom * 

 has often been likened to a solar system where the sun 

 corresponds to the nucleus and the planets to the 

 electrons. The analogy, however, must not be pressed 

 too far. Suppose, for example, we imagined that some 

 large and swift celestial \-isitor traverses and escapes 

 from our solar system without any catastrophe to itstlf 

 or the planets. There will inevitably result permanent 

 changes in the lengths of the month and year, and our 

 system will never return to its original state. Contrast 

 this with the effect of shooting an electron or a-particle 

 through the electronic structure of the atom. The 

 motion of many of the electrons will be disturbed by 

 its passage, and in special cases an electron may be 

 removed from its orbit and hurled out of its atomic 

 system. In a short time another electron will fall into 

 the vacant place from one of the outer groups, and this 

 vacant place in turn will be filled up, and so on until 

 the atom is again reorganised. In all cases the final 

 state of the electronic system is the same as in the 

 beginning. This illustration also serves to indicate 

 the origin of the X-rays excited in the atom, for these 

 arise in the process of re-formation of an atom from 

 which an electron has been ejected, and the radiation 



