A Periodic Table Based on Atomic Number and Electron Configuration Where to Place Th, Pa, and U in the Table JOSEPH A. BABOR Colkge of the City of New York,New York City
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F. LUDER presented a periodic table based on electron codguration in the JOURNAL OF CHEMICAL EDUCATION (January, 1943) in which the order of atomic numbers is interrupted. The table presented in this article is a modification of Luder's. It is not necessary to interrupt the order of atomic numbers; in fact there is much to be gained by retaining it, for i t is then a simple matter to follow the change in electron configuration as one proceeds from one element to the next, without skipping around the table. The complete electron configuration for each element
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X e Related
is given by the figures in the vertical column a t the right of the symbol; the energy levels are indicated a t the left of the first element of each series. The atomic number appears above the symbol. There are 32 columns in this table. The first 8 include the Representative Elements; columns 9-18 contain the Related or Transitional Metals; and columns 19-32 make up the Rare Earth Elements. The number a t the top of each column represents the number of electrons in the energy level where the differentiating electron is located. In the first class, the representative or 5 a n s i t o o n d
Metals.
J i e Rare Earth Elcmenl
elements, the differentiating electron is in the highest energy level; hence the column number indicates the number of electrons in the highest energy level of the elements in the respective column without exception. In the second class, the related metals, the column number represents the number of electrons in the second from the highest energy level; M in the 4th period, N in the 5th, 0 in the 6th, and P in the 7th period. To further indicate this a double-line border encloses the series in this class. Examination of the chart shows that there are a few exceptions to this rule, i. e., Cr, Cu, Cb, Mo, Ru, Rh, Pd, Ag, Ir, Pt, and Au. In the third class, the rare earth elements, the column number indicates the number of electrons in the third from the highest energy level, N in the 6th period, with no exceptions. This is also shown by the triple-line border. In this chart the elements in a given period are not separated as in the Luder Chart, yet it has all the advantages of the Luder arrangement. A glance a t the chart shows a similarity in arrangement for periods 2 and 3, and another for 4 and 5. Note particularly the sequence and positions of the
elements in periods 4 and 5. The gap in the atomic number sequence in the first class, 20 to 31 and 38 to 49, is filled by the series in the second class. When we consider period 6, the large gap, 56 to 81, in atomic number sequence in the representative elements makes allowance for the intemupted series in the related elements, which also has a gap (57 to 72) that in turn allows for the series in the rare earth elements. It would appear from the fonn of the table that there should be a repetition of this double interruption in period 7. Perhaps the elements Th, Pa, and U are out of place and should be placed in the third class, the rare earth elements, under Ce, Pr, and Nd, respectively.' In this way period 7, although incomplete, would follow tbe same pattern as period 6 and carry out the general plan of the chart. This would mean that the electron configurations would have to be changed to Th, 2, 8, 18, 32, 19, 9, 2 ; Pa, 2 , 8 , 18, 32, 20, 9, 2; and U, 2, 8, 18, 32, 21, 9 , 2 . V n L A R , "A suggested revision of the position of thorium in the fourth period of the periodic table," J. CHEM.EDUC..19, 329
(1942).
