Page:Popular Science Monthly Volume 87.djvu/140

136 have all the nucleus charges from 1 for hydrogen to 92 for lead. There is, however, another very important consequence that follows from this conception of the atom. Disregarding for a moment the atomic weight which depends mainly on the structure of the nucleus, the main physical and chemical properties of the atom are determined by the nucleus charge and not by the atomic mass. This must obviously be the case, for the number and distribution of electrons round the nucleus is determined by the electric forces between the electrons and the nucleus, and this is dependent on the magnitude of the nucleus charge which may be regarded as a point charge. Without entering into the difficult question of the actual distribution of the exterior electrons in any atom, it is obvious that the number and position of the outlying electrons of the atomic structure, which probably mainly influence the chemical and physical properties of the atom, are determined by the charge on the nucleus. No doubt if the electrons are in motion, their positions relative to the nucleus and possibly also their rates of vibration will be slightly influenced by the mass of the nucleus as well as its charge, but the general evidence indicates that this effect must be very small.

We thus see that there is in the structure of every atom a quantity which is more fundamental and important than its atomic weight, viz., its nuclear charge. It is known that the variation of the atomic weights of the elements with atomic number, while showing certain well-marked relationships, shows no definite regularity. From the point of view of the nucleus theory, the atomic weight of an element, while in some cases approximately proportional to its atomic number, is in reality a complicated function of the actual structure of the nucleus. The question why the atomic mass should not necessarily be proportional to the atomic number has already been discussed. While the main properties of an atom are controlled by its nuclear charge, the property of gravitation and also that of radioactivity are to be ascribed mainly, if not entirely, to the nucleus.

Since the nucleus charge of an atom determines the main physical and chemical properties of an atom, it is possible that elements may exist of equal nuclear charges but different atomic weights. For example, if it were possible to add a helium nucleus to the nucleus of another atom, it would increase the nuclear charge by two and the mass by about four; if instead of the helium nucleus two hydrogen nuclei were added, the charge would be the same but the mass of the resulting atom two units less than with helium. In such a case, two atoms would be possible of identical nuclear charge but different atomic weights. In a similar way, it may be possible for elements to exist of the same atomic mass but different nuclear charges. This would be brought about by the loss or gain of one or more negative electrons in the nucleus.