Letters to the editor
“The” periodic table
My late father, Henry Bent, delighted in chemical periodicity and ways to depict it and often said there is no place for the article “the” in front of “periodic table”: “Which periodic table is best? is like asking Which table utensil is best?” he says in his book New Ideas in Chemistry from Fresh Energy for the Periodic Law. Earlier in the book, he writes, “The question [of which table is best] is as impossible as unnecessary to answer.”
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Bent recognized, however, that the shape of Charles Janet’s left-step periodic table is a powerful tool for assigning the problem elements to their correct positions in the periodic system and most notably for promoting helium to its proper spot over beryllium. “Two things are surprising about Mendeleev’s classification of the chemical elements according to the Periodic Law,” Bent writes. “One is how much of the classification has not needed revision. The other is how long the scheme’s second element has been misplaced, above neon, despite Mendeleev’s Rule of Light-Element Distinctiveness; despite his statement that the Periodic System is about atoms, not simple substances; despite classification by atomic physics of helium atoms as s2 systems (not p6 systems); despite appearance of the Left-Step Periodic Table nearly eight [now over nine] decades ago; and despite numerous implications of the LSPT that require placement of helium above beryllium.”
Classifying helium with neon is an artificial classification of simple substances, while classifying helium with beryllium is a natural classification of atoms. Mendeleev noted that both types of classifications are useful, but only the periodic classification of atoms is a natural classification. (The phenomenon of allotropy implies that the periodic classification of the elements is not a classification of simple substances, such as diamond or graphite, oxygen or ozone.) Helium above neon negates many of periodicity’s regularities: it violates the rules of triads, group sizes, first-element distinctiveness, spectroscopic classification of atoms, and atomic structure. In its ionization energy and electronegativity, helium stands to beryllium as (to a good approximation) hydrogen stands to lithium. Not one to understate, Bent writes, “The Mendeleev-Jensen Block-to-Block Trend in Light-Element Distinctiveness . . . is, nonetheless, arguably, the most general feature of the chemistry of the elements after the Periodic Law itself.
“The Mendeleev-Jensen Rule, together with the electronic interpretation of periodic tables, makes helium-above-neon one of the leading contradictions in the history of chemical thought.”
Libby Bent Weberg