The occurrence of elements of different chemical properties, but the same mass, the so-called isobares (see ISOTOPES), also indicates a difference of nuclear charge with no difference of nuclear mass, which can be best explained as the effect of a difference in number of electrons in the nucleus. For the addition of an electron to the nucleus will diminish Z by f, but not ap preciably change the mass. The existence of isotopes (q.v.) is also easily explained upon the hypothesis that the nucleus is ultimately built up of protons and electrons. Isotopes are ele ments which have the same chemical properties, that is, the same Z, but different masses. If, to a given nucleus we add n protons and n electrons we do not change the charge, but increase the mass by n units.
It being established that the nucleus consists of protons and electrons, the question arises as to whether any special com binations exist as sub-groups within the nucleus. The expulsion of a-particles by radioactive nuclei suggests that, inside heavy nuclei at any rate, we have particles composed of four protons and two electrons, existing as particularly stable units. The stability of the a-particle is further discussed in the last section of this article. Meitner has further invoked the existence of par ticles composed of four protons and four electrons to explain certain peculiarities of the transformations of the radioactive families. (See RADIOACTIVITY.) Two of the electrons are assumed to be less firmly attached than the other two, so that the com bination is really a neutralised a-particle. When one electron leaves such a neutralised a-particle, the rest is assumed to become unstable, so that a sequence of changes is to be expected, in which two 0-ray emissions and one a-ray emission are associated in some order or other. In the general run of the radioactive transformations this does, in fact, occur, as, for example, where Thorium emits an a-particle, changing to Mesothorium I, followed by two (3-ray changes, or where Uranium Y is transformed with a 0-emission, to be followed by an a and a 0 change. The branching of the radioactive series (such as follows, for example, the trans formation of Radium B), where a (3 emission is followed either by a (3 and an a change, or alternatively by an a and a 0 change (see RADIOACTIVITY) is also explicable on Meitner's hypothesis.
The hypothesis that the nucleus of radioactive atoms contains a-particles, each neutralised by two tightly bound electrons, was also used by Rutherford to explain the origin of the a-rays. The a-particle, with its double positive charge will be strongly repelled if it once be released from whatever forces may be supposed to bind it in the nucleus. We can calculate the energy with which the particle will leave the atom, supposing it to be all gained by virtue of the work done on it by the nuclear field of force, if we know the distance from the centre of the nucleus at which it starts. Thus the charge on the rest of the nucleus being Z'e, while 2e is the charge on the a-particle, this energy is which, taking the slowest a-particle, that from Uranium I, with energy 4.07 X f electron-volts, gives the value 6.3 X cm. for r, and any assumption that the a-particle possesses an initial velocity would lead to an increased value of r. Scattering experi ments, however, show that the inverse-square law holds for dis tances as small as 3 X cm., which could not be the case if there were particles with two positive charges in the region of the nucleus indicated by the above estimate. If, however, the a particles at distances 6X cm. from the centre of the nucleus normally exist in a neutralised state they will have no effect on the scattering, and the two estimates can be reconciled. The electrons which neutralise the a-particles cannot, of course, exist in the kind of outer orbits which they occupy for the neutral helium atom, for there would be no room for them, but must be held in some much more intimate manner. Rutherford's sup position is that the neutral a-satellite, as we may call it, is attracted to the central bulk of the nucleus by the polarisation forces set up in it by the nuclear field, and that, controlled by these forces, the satellite executes a quantum orbit, something like that of an electron in the outer parts of the atom. (See Mom.) When the satellite loses, by some unknown mechanism, its two electrons, it is repelled, gaining its final velocity of expulsion in its passage to confines where the nuclear field is negligible. The electrons are retained by the nucleus, and ultimately discharged in 0-ray charges. A sketch to illustrate this conception of the radioactive nucleus is given in fig. 3.