s block

The s-block elements include the unique metallic gas hydrogen and the noble gas helium, which fill the K shell with 1s electrons, and the alkali metals and alkaline-earth metals, whose outer shells contain, respectively, one or two s electrons. At least one notable authority, John Emsley (The Elements, Third Edition with corrections, Oxford University Press, 2000) divides the s-block into two, treating hydrogen and helium as a separate 1s block as a mark of their uniqueness. Under traditional iterations of the periodic table, in which designers have never agreed where hydrogen should be placed, this seems a useful convention, as hydrogen has been variously placed (1) above the alkali metals; (2) above the halogens and adjacent to helium; (3) floating free in the first period; or (4) in two positions, above both helium and fluorine. The Periodic Spiral relies on its improved design to demonstrate the relationship between hydrogen and the halogens, the noble gases, and the alkali metals, and therefore features a single s-block.