There is one element, however, besides lead, about which interesting evidence has been obtained on this point. Sir J. J. Thomson found by examining the deflection of the positively charged particles produced by an electric discharge through the rare gas, neon, that two elements were present of atomic weights about 20 (neon) and 22. Aston was able by diffusion experiments to separate partially the two components of neon and to show that they differed in density, but failed in attempts to separate them by fractional distillation in charcoal cooled by liquid air. Such results are to be anticipated if neon is a mixture of two isotopes, i.e., elements of identical nuclear charges but different atomic weights.
It is obvious that this new point of view will result in a systematic examination of the elements to test for the possible presence of isotopes, and thus give an additional reason for the accurate determination of atomic weights for elements obtained from widely different sources.
Distribution of Electrons in the Atom
It is seen that the nucleus theory of the atom offers a simple explanation 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 difficulties; for an electron is attracted towards 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 simplest 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 towards the positively 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 which is connected with the difference of potential energy E of the electron in the two stationary states by E·h where h is Planck's fundamental constant. On this hypothesis, he has been able to account for the series spectra of hydrogen and to deduce directly from the theory the value of Balmer's constant which plays such an important
