Closing periods with the alkali metals gives to the completion of an s subshell

Closing periods with the alkali metals gives to the

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Closing ‘periods’ with the alkali metals gives to the completion of an s subshell a finality that is completely absent from the behaviour of the elements. The electrons in the outermost s orbital are available for bonding along with electrons subsequently added in a p, d or f orbital. Although the build-up of d or f subshells is not always regular, their completion is final as that of an s subshell is not. On the other hand, the most striking periodic feature of the entire sequence of elements is the peculiar completeness of the eight electrons of the combined s and p subshells, or of 1s alone where there is no 1p. Within the rows, tables encourage chemists to divide rows into ‘blocks’ based on the filling of subshells, lending these convenient fictions a specious reality. This has given rise to lengthy arguments about where to place, for example, zinc, cadmium and mercury (Jensen, 2003), whether lanthanum or lutetium belongs in the f block (Jensen, 1982), and 6
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[ FoC p. 240] Figure 3: Janet’s left-step system in its spiral form. whether aluminium belongs with scandium rather than gallium (Habashi, 1997). A pragmatic but untidy solution is to place some elements in two different positions (Laing 2005). THE POTENTIAL OF SPIRALS Such difficulties arise only because the sequence has been chopped up to make a table. Representation of the system as a spiral dispenses with the need to decide where ‘periods’ and ‘blocks’ begin and end. It shows every element between its neighbours in the continuum. By joining the ends of the table together, it makes it possible to integrate the lanthanides and actinides without the image becoming excessively wide. Colours can be used to indicate affinities such as those of the n and n+10 groups and those of lanthanum, gadolinium and lutetium. 7
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The spiral also facilitates the placing of hydrogen, which in a table usually heads either the alkali metals or, going to the opposite extreme, the halogens. Alternatively it can float over the middle of a table without belonging to a group, as advocated by Atkins and Kaesz (2003). However, hydrogen has far more in common with carbon than with any other element. Each is half way to a full outer shell, and they are the only elements that have the same number of valence electrons and valence orbitals. They have medium electronegativity and favour covalent bonding, forming the strongest single bonds between like elements [ FoC p. 241] as well as very strong bonds with each other. These and other arguments persuaded Cronyn (2003) to place it in group 14. This looks awkward in a table, but hydrogen sits comfortably next to carbon in a spiral. A spiral also poses the intriguing question of how to start the sequence. It is possible to envisage an ‘element of atomic number zero’ with no protons and no electrons 7 .
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