Discussion on isotopes

Abstract

Sir J. J. Thomson: I think I shall best open the discussion this afternoon if, instead of attempting a history of the subject of isotopes, I confine myself to a few special points. The conception of isotopes—that is, of two elements which have different atomic weights but chemical properties so closely resembling each other that they have not yet been separated by chemical methods—was, I believe, first reached by Prof. Soddy in connection with radio-active substances. In the few remarks which I have to make this afternoon, I shall not deal with these substances; I will leave them to others who are better acquainted with them than myself; I propose to confine myself to the consideration of the isotopes, or with some points connected with the isotopes, of the lighter elements. Before doing so, I should like to say just a word about the conception of isotopes on the electron theory of an atom. On that theory the atoms of the isotopes contain an equal number of electrons. The difference in the atomic weight is supposed to be due to the simultaneous entry into the core of the atom of one or more positive charges, and an equal number of electrons, so that the electric charge on the core is not affected. In reference to this, I would like to say that it by no means follows that an electron and a positive charge will neutralise each other at the distances that occur in the atom, however close together they might be placed; and, though one would expect that the residual effect would not be large, yet I think it might easily be appreciable, and might produce some difference between the properties, chemical or physical, of the isotopes. For example, though I do not think it would affect the number of electrons in the outer layer, I think it would affect the distance of the outer layer from the core of the atom. And the statement that the chemical properties of an element depend only on the outer layer of the atom, is one with which I should agree if it is recognised that, even if the number of electrons in the outer layer is unaltered, differences in the distance of this layer from the centre may result in different chemical properties. For example, we, or at any rate some of us, believe that the valency of an element fixes the number of electrons in the outer layer. Thus, there are the same number of electrons in the outer layer of the atoms of silicon and of carbon, and yet there are well-marked differences in the chemical properties of these two substances.