ON A SUGGESTED REVISION OF THE SEVENTH PERIOD OF THE PERIODIC TABLE' G. E. VILLAR University of Monteddeo, Montevideo, Uruguay
I N THE generally accepted classification of the Periodic Table, elements 87, Ra, Ac, Th, Pa, and U, which make up the seventh period, have been placed successively and occupy subgroups (a) of Groups I, 11, 111, IV, V, and VI. No objection can be raised regarding the location of the first three elements of this period; on the other hand, the analogies characteristic of the Periodic Table cast some doubt as to the location of the following elements. In the first place, the similarity of periods 2 and 3, 4 and 5, suggests a possible analogy between periods 6 and 7. This analogy is confirmed if account is taken of the sequence of atomic numbers the gases, in which the difference between two consecutive atomic numbers corresponds to the number of elements tained in the ~ e r i o dbetween them. This sequence is made up of the numbers:
Bearing in mind Pauli's exclusion principle, and expanding the previous expression, we have for this element the following electron configuration which coincides with that suggested by Bohr for krypton: + + =E ,+ ~ + + N,* + +
+ N",
Mp8
MP
If we calculate by means of fornula the atomic number of the element which would complete the seventh period, we obtain 2 = 118
which coincides with that found by ~ ~ forhther nuclear charge of the seventh noble gas. This conclusion, is based on the closesimilarity which characterizes all noble gases, leads to the assumption that period seven has 32 elements, just as the sixth period. If the sixth and seventh periods are made up of an equal number of elements, they should have identical configura2, 10. 18, 36.54.86, . 2. tions and therefore there must exist in the seventh pewhere Z, is the atomic number of the noble gas a t the riod an of 15 elements similar to the rare earths end of the pthperiod, and which, by analon, should occupy the place reserved so Z, = Z p ( Z X l - 1 ) + 2 ( 9 - l ) ( Z X Z - l ) + Z @ - 3 ) far for actinium alone (Z= 89): (2~3-1)+2(p-5)(2X4-1)+...+ On the other hand, J. Perrin, bearing in mind the '(9 - 2n + 3)(2n sirnilaritv between lanthanum and actinium. has surThis expression, in which only the positive terms should gested the possibility that in place of actinium there be considered, corresponds to the law of formation of should exist a second series of rare earths. The probthe extra nuclear electronic shells of the noble gases in able existence of this new array of elements will force a accordance with Bohr's ideas, where the numbers 1, 2, revision of the final part of the Periodic Table. If it is accepted that the atomic numbers of thorium 3, 4, correspond to the terms s, p, d, f, . . . For example, the fourth period would be completed with (Z = go), protoactinium (Z = 91), and uranium (Z = 92), are correct, then i t would be necessary to the element of atomic number Z4, i. e., move these elements to the place now occupied by Z4 = 8. 18, 106 = 36 actinium, while if the present location of such elements ' CJ V I L L ~Ann. , Acad. Brasil.Sci.,12, 51-7 (1940); MAYER,is correct, then their atomic numbers should be raised by 14 units. Phys. Rev., 60, 184-7 (1941).
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