i 4 o TEE POPULAR SCIENCE MONTELY 



part in the spectra of all atoms. In a similar way, the helium atom 

 is supposed to consist of a nucleus of two unit charges surrounded by 

 two electrons. On the theory, the spectrum of helium is connected in 

 a very simple way with that of hydrogen. Bohr also pointed out that 

 the Pickering series of spectral lines observed in certain stars which 

 were originally attributed to hydrogen must be ascribed to helium. 

 This conclusion has since been strongly supported by the direct experi- 

 ments of Fowler and Evans, In a similar way, Bohr described the 

 possible distribution of electrons in several of the lighter atoms and 

 also discussed the structure of the hydrogen molecule, which is com- 

 posed of two hydrogen nuclei and two electrons. The heat of com- 

 bination deduced for the theoretical molecule is in fair accord with 

 experiment. He found that two helium atoms were unable to unite to 

 form a molecule — in agreement with a well-known property of this gas. 



While there is room for much difference of opinion as to the in- 

 terpretation of the rather revolutionary assumptions made by Bohr to 

 explain the structure of the simple atoms and molecules, there can be 

 no doubt of the great interest and importance of this first attempt to 

 deduce the structure of the simple atoms and to explain the origin of 

 their spectra. The agreement of the properties of such theoretical 

 structures with the actual atoms is in several cases so remarkable that 

 it is difficult to believe that the theory is not in some way an expression 

 of the actual facts. While much work will be necessary before we can 

 hope to understand the structure of any but the simplest atoms, a 

 promising beginning has been made in the attack on this most difficult 

 and fundamental of problems. 



There seems to be little doubt that the more marked physical and 

 chemical properties of an atom are to be attributed to a few outlying elec- 

 trons in the atomic structure. The position and number of these " val- 

 ency " electrons, as they have been termed by Stark, are defined by the 

 magnitude of the nucleus charge. It has previously been pointed out that 

 the loss of an alpha particle from a radioactive atom changes the posi- 

 tion of the element two groups lower in the periodic table, while the 

 loss of a beta particle raises it one group higher. Consequently it fol- 

 lows that the loss or gain of a unit charge from the nucleus of an 

 atom causes it to change its position from one group to the next. If, 

 for example, we follow the chemical properties of successive elements 

 when the nucleus charge increases by unity, we soon reach an element 

 which belongs to the same group as the first, although of much higher 

 atomic weight. We must consequently conclude that the number and 

 position of the outlying electrons in the structure of the atom passes 

 through successive changes which are regularly repeated with increasing 

 atomic weight. Quite apart from any detailed knowledge of the elec- 

 tronic distribution of atoms, the regular recurrence of elements of 



