science Thursday, April 1, 2010. Post by HeBe
The history of the periodic table has been full of surprises. Discovery of the Inert Gases, an entirely new Group, was one, for instance. Discovery that the “Inert Gases” aren’t actually inert is another; today they are known as the noble gases. The discovery of the trans-uranium elements and the recognition of a whole new Period, the Actinides, was yet another. Controversy rages on even the basic format of the Periodic Table.
But, one question has always puzzled Henry Bent. Is helium, He, in the correct place in the standard Periodic Table that adorns thousands of science classrooms and laboratories across the globe, or should it really be place above the alkaline earth metal beryllium, Be, next to hydrogen, H?
In this guest post, Henry Bent, HeBe, offers an answer.
Helium seems to be in a perfectly natural location where it is, above neon. Both are “inert” gases. And throughout most of the Periodic Table similarity and adjacency are companions. In the s-block there’s sodium above potassium and in the p-block there’s arsenic above antimony, selenium above tellurium, chlorine above bromine, and krypton above xenon. In the d-block there’s titanium above zirconium and zinc above cadmium and in the f-block cerium above thorium. Furthermore, He Ne Ar is a perfect triad. Case closed? Not quite.
The argument against helium-above-neon:
Breaks twice-over the First Rule of Triads: A Group’s first member is never the member of a primary (vertical) triad. The Rule holds for 30 of a periodic table’s 32 Groups when He is above Ne. The two exceptions are He Ne Ar and Be Mg Ca. The Rule holds for all 32 Groups if He is located above Be.
Breaks the rule for two of periodic tables’ 32 columns that elements’ ordinal numbers within their Groups, starting at unity at the top, are equal to the number of radial nodal surfaces r, counting ones at infinity, in the predominant type of orbitals occupied across a block in Bohr’s Aufbau Process. For the s-, p-, d-, and f-blocks those orbitals are s-, p-, d-, and f-orbitals, respectively. With He above Ne, rather than above Be, the ordinal number of, e.g., Ne, a 2p6 system, r = 1, is 2, and the ordinal number of Be, a 2n2 system, r = 2, is 1. With He above Ne, the Ordinal Number Rule does not hold for any of the elements of the Ne and Be Groups. With He is located above Be, the Ordinal Number Rule holds for all elements in the Periodic System.
Breaks twice-over the Rule of First-Element Distinctiveness: Groups’ first-elements are particularly distinctive with respect to their congeners, with distinctiveness increasing, block-to-block, in the Jensen order s >> p > d > f. With He above Ne, the rule does not hold for He, nor does it hold for Be of the s-block more so than for first-elements of the p-block. With He above Be, the rule and the Jensen order holds for all first-row elements.
Does science want to live forever with these inconsistencies in its iconic representation of chemistry?
The Rule of First-Element Distinctiveness has two dimensions to it: vertical and horizontal. The vertical dimension is a block-to-block trend in distinctiveness of first-row elements with respect to their congeners. The horizontal dimension is a block-to-block trend in distinctiveness of first-row elements with respect to their first-row neighbors. He/Ne destroys both of those trans-table trends.
Two new trans-table, block-to-block trends would not be particularly significant for the Periodic System if periodic tables exhibited already many trans-table trends. In fact, however, there is only one other recognized trans-table trend. It’s exhibited by sfdp tables through four complementary well-known trends, left-to-right:
Active metals through metalloids to nonmetals and the Noble Gases
Basic oxides through amphoteric oxides to acidic oxides
Reducing agents to oxidizing agents
Runs through maximum oxidation numbers, beginning at +1
The four trends reflect a transition from large cores with small charges (e.g., r = 95 pm for Na+) to small cores with large charges (r = 26 pm for Cl+7).
Cores’ sizes and charges are the first stop in understanding all of descriptive chemistry.
The renowned chemical educator J. Arthur Campbell was famous for, among other things, his display of atomic cores in the format of a periodic table.
There is also the well-known trend downward in all Groups toward increasing metallic character with increasing core size: e.g., for the nitrogen Group, from nonmetallic nitrogen, r = 11 pm for N+5, through the metalloid arsenic, r = 47 pm for As+5, to metallic bismuth, r = 74 pm for Bi+5.
The downward nonmetal-to-metal transition is particularly pronounced, at the outset, in the s-block, for H above Li and He above Be. That onset is less pronounced in the p-block, and still less so in the d- and p-blocks, where all elements are metals. The phenomenon exhibits a Jensen-like block-to-block trend, increasing in the order: s >> p > d > f. Therein is another trans-table trend contingent on location of helium above beryllium.
In summary: helium’s location in periodic tables is a big deal. Above beryllium, helium stands out as the Periodic System’s most distinctive element. (Above neon helium does not stand out as a distinctive element.) Above beryllium, helium anchors a number of previously unrecognized regularities in the Periodic System. Helium-above-neon conceals the new regularities.
Of all the elements, no element’s location is more important for the introduction of new ideas into the Periodic System than helium’s location. (Imagine being without He!) Surely it’s important to locate the Periodic System’s most important element in its truly natural location.
Helium above beryllium brings out the best in the Periodic System.
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