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Table the Discussion

To the Editors:

I am surprised that in Eric R. Scerri's otherwise informative article about the periodic table ("The Past and Future of the Periodic Table," January-February), an important underlying principle is not mentioned. With reference to the valence electron configuration of the ground state of each element, the table can be viewed as consisting of rectangular blocks labeled by the azimuthal quantum number for the least stable electron (called s, p, d, f). Further, each column of the table can be labeled with the number of valence electrons sharing that azimuthal quantum number, and each row of the table can be labeled with the highest principal quantum number found among the electrons. Only helium, which is unique because its s2 valence configuration also is a closed shell, is "out of place" in this labeling scheme.

Labeling in this way is more than just place-holding. Inasmuch as the valence electrons are the most important determinant of the chemical properties of an element, this feature is the underlying explanation for periodicity. In addition, the table itself becomes an important mnemonic tool for students.

The left-step table can also be divided into analogous blocks and its columns labeled analogously, but its rows do not match up with highest principal quantum number (it changes from 2 to 3 between Ne and Na, and so on). Dr. Scerri's proposed table not only shares this disadvantage but also divides the p block awkwardly. Moreover, periodic trends such as first ionization energy and atomic size undergo striking changes between elements of group 18 (noble gases) and group 1 (alkali metals), yet these groups are adjacent in Dr. Scerri's proposed table.

As to the desirability of getting rid of the "implied" breaks between periods in the conventional table, both atomic radius and first ionization energy change immensely between any noble gas and its immediate successor alkali metal. That is certainly a real break in properties.

John Olmsted
Pittsburgh, PA

Dr. Scerri responds:

Dr. Olmstead suggests that the conventional, or medium-long form table, more clearly reflects the order of electron shell filling than the left-step form table. However, the principle of electron-shell filling follows an increasing rule for the quantum numbers n + l and not an increasing n rule. Admittedly there is a change in n from 2 to 3 as one crosses from Ne to Na in the left-step table, but this is better, not worse, than the change in n from 4 to 3 and back to 4 as one crosses, say, the fourth row in the conventional table. And if one considers the long-form table based on an increasing value of n for each new row the situation is even worse. For example, in period 6 we have n going from 6, to 4, to 5 and back again to 4. At least in the left-step form the values of n show a consistent increase on moving from left to right.

I have published a proposed table without a division in the p block in the April issue of Journal of Chemical Education. My proposals are more concerned with the problematic placement of the elements H and He rather than mere shape of the table. In doing so I seek an objectively "more correct" classification of H and He into existing groups. Seeing an overall increase in ionization energy as one crosses the periodic table is just a conventional aspect that we have become accustomed to and nothing more.The criteria that I use in seeking an objective location for H and He involve atomic number triads, as explained in my article. This is because atomic numbers and relations between them are categorical criteria for building up a periodic table whereas electronic configurations show a number of anomalies such as the ones alluded to by Dr. Olmsted. More importantly, electronic configuration does not settle where to place H and He.

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