CONSTITUTION OF MATTER RUTHERFORD. 199 



DISTRIBUTION OF ELECTRONS IN THE ATOM. 



It is seen that the nucleus theory of the atom offers a simple ex- 

 planation of many important facts which have been brought to 

 light in recent years, and for this purpose it has not been necessary 

 to make any special assumptions as to the actual structure of the 

 nucleus, or of the way in which the external electrons are distributed. 

 The investigation of the latter problem is beset with many diffi- 

 culties; for an electron is attracted toward the nucleus, and even if 

 it is in orbital motion, it must, on the electromagnetic theory, lose 

 energy by radiation and ultimately fall into the nucleus. It appears 

 likely that this difficulty is in reality due to our ignorance of the 

 conditions under which an electron radiates energy. According to 

 the views outlined in this lecture, the hydrogen atom has the sim- 

 plest possible structure, for it consists of a nucleus of one unit 

 charge and one negative electron. The question naturally arises 

 how such a simple structure can give rise to the complex spectrum 

 observed for hydrogen. This problem has been attacked in a series 

 of remarkable papers by Bohr, who concludes that the complexity 

 of the spectrum is not due to the complexity of the atomic structure 

 but to the variety of modes in which an electron can emit radiation. 

 Suppose, for example, that a hydrogen atom has lost its negative 

 electron. Bohr supposes that an electron falling toward the posi- 

 tively charged nucleus may occupy temporarily any one of a number 

 of stationary positions fixed relatively to the nucleus. In falling 

 from one stationary state to another, radiation is emitted of a definite 

 frequency v which is connected with the difference of potential energy 

 E of the electron in the two stationary states bj^ E=h v where h is 

 Planck's fundamental constant. On this hypothesis, he has been able 

 to account for the series spectra of hvdrogen and to deduce directly 

 from the theory the value of Balmer's constant which plays such an 

 important 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 this 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 sup- 

 ported by the direct experiments 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 composed of two hydrogen nuclei and two elec- 

 trons. The heat of combination deduced for the theoretical molecule 

 is in fair accord with experiment. He found that two helium atoms 



