VOL.2, NO.2
A MODELOP TEE PERIODIC TABLE
107
A MODEL O F THE PERIODIC TABLE* M. COURTINES. LABOEATORY OP EXPERIMENTAL PHYSICS, COLLEGE OP FRANCE, PARIS
As is well known, the basis of the table is the remarkable but not altogether complete periodicity of the chemical properties (Mendeleeff) and of certain physical properties (Lothar Meyer) of the elements. The nature of our ideas on the subject has now completely changed. It is the number of electrons exterior t o the atomic nucleus, equal to the charge of the nucleus, taking the charge of the electron as the unit, which determines the arrangement of the elements in a definite order. This number is furnished by physical measurements, visible, ultra violet and X-ray spectra, deflection of a-particles, etc. This leads t o a classification of the elements by their successive layers of electrons, called K, I,, M, N, 0 , P, Q; i t is the formation of these successive layers which is a t the foundation of the periodicity. The accompanying table (Fig. 1) shows: 1. The arrangement of the electrons in successive layers. 2. The chemical relationships. 3. The physical relationships. It is given in the form of a card model, which may be cut out and glued together, as is shown in Fig. 2. Each square contains the symbol of an element in large black characters. Above and to the left is given the ordinal number, Z, which is the result of physical measurements. The card is folded in such a manner that elements 5 and 13 (B and Al) follow elements 4 and 12 (GI and Mg). For this purpose the card is cut between Al, Si and Ga, Ge. Elements 26, 44, 76 (Fe, Ru, 0s) are folded . , ,...% ! ' ~ / , _,, and glued, back to back, against the elements 25, 43, 75. Elements 28, 46, 78 (Ni, Pd, Pt) are glued in the same manner to the backs of the elements 29, 47, 79 (Cu, Ag, Au). The rare metals, folded like an accordion, and the are attached backs Ta) between of elements by gluing which 56their there and ends 73 is (Ba left to
n Fix. 2.
a little window to receive the whole series of rare metals. Hydrogen (Z= 1) is folded and glued to the back of helium 2). was obtained through the courtesy of W. H. Koyes. Univenity of hi^ Illinois, Urbana, Ill.
(z=
When glued together in this manner, the model forms a tower, which is tinted gray, with a basement of white attached. This is shown in Fig. 2. The numbers a t the bottom of each square (to be given in red in the cardboard model) give the division of the extra-nuclear electrons in successive layers of the atoms. The numbers (to he also in red) a t the left of the symbols give the atomic weights. There is also given in green a t the right in each square the physical atomic weights as given by the mass spectra (Aston). As these numbers cannot be brought out clearly in the scale used for the whole, the following illustrations are given t o show what is intended:
The tower, whose middle face bears the noble gases, carries the elements which show close chemical relationships. It must have been these elements which guided the brilliant imagination of Mendele&. The elements grouped in the annex are too far away from the normal arrangement of the noble gases for us to assign t o them special chemical characters. In this zone the chemical periodicity, dear to Mendeleeff, disappears. But the physical periodicity remains. Column 24, 42, 72, 92 is that of the refractory metals. Column 29, 47, 79 is that of metals of high conductivity. In general, when an electron is added t o the exterior of the atom i t has no effect on the interior layers except that the energy of the trajectory is increased. In the region 26, 27, 28, 44, 45, 46, 76, 77, 78 when an electron is added it falls to an interior layer; these layers are no longer inviolable; they are modified, not only in the number of electrons but also in-their electronic value. They are called "continuing" (remaniement). The addition of an electron does not increase the outer layer but remains within the edifice.
,'Refrain from illusions, insist on work and not words, Patiently search divine and scientific truth."
MARIAMENDELEEF
