Discontinuous tables

Attractive as the continuous tables are, the discontinuous tables have been undoubtedly more numerous and popular. Discontinuities have been introduced in various ways, and some of these tables appear at first glance to be almost of the continuous kind, such as the one drawn by Sheele in 1950. The analogy with electron orbits around a central nucleus is an added attraction of this table, but closer inspection shows that certain elements do not follow in atomic-number sequence - nor can they in a table which is arranged in order of the principal quantum number.

Discontinuous tables are almost invariably of a linear format and with atomic numbers increasing from left to right and down the table. All these features are so taken for granted as no longer to be questioned, but we should not forget that Mendeleev ‘s first table, though discontinuous, has the elements arranged in horizontal groups with atomic weights increasing from top to bottom. However, his original paper also contained a table with the groups arranged in vertical groups, as in the common type used today.

The reason why this particular format has triumphed is probably due to the Western way of reading in which we can scan from left to right and top to bottom. This familiar eye movement obviously will encourage a table designed to conform to it. We need look no further to explain why the standard periodic table has always been of this form, whether it has been of the 8-, 18- or 32- group format.

With discontinuous tables there need to be defined discontinuities, and early tables had no special reason for ending a row with any particular group. The earliest 8-group tables in which elements were grouped according to ‘valency’ naturally started at valency I and ended at valency VIII. Such tables however, brought together elements sharing a common valency but little else. Thus it became necessary to have subgroups A and B in order to make chemical sense of the groups. This A and B terminology was eventually to lead to a conflict between periodic tables that was only finally resolved in 1985 by the intervention of the International Union of Pure and Applied Chemistry.

With the discovery of the so-called noble gases, the obvious place for them was in a group O on the left hand side of the table because these elements were considered to be chemically inert until Bartlett’s discovery of the first xenon compound in 1962. However, the noble gasses were soon to occupy a special place in the theory of chemical bonding, their inertness, and this idea of filling electron shells as one moves along a row on the right and marked the end of a particular row.

The long form of the periodic table with the modern format has been attributed to H G Deming, who devised it for his textbook in 1923. Its popularity was ensured when it was adopted by a drug company as part of their promotional material. This gave it wide publicity, but clearly it met a need, and it gradually ousted all others until today it reigns supreme. However, it has a longer ancestry than this; 18-group tables can be traced back to the 19th century - even Mendeleev came up with a version, in his case based on 17 groups, of course.

The modern table is not content solely to list the elements in rows ending with the noble gasses, but fragments the table into five blocks accordingly to the type of orbital shell which is being filled. The structure of the table is now taught in terms of the four quantum numbers n, l, m and s. The rows ending with a block are based on the principle quantum number n, with its integral values 1, 2, 3 and so on. The blocks are called after the orbital quantum number 1, which can have the numerical values 0,1,2 . . . n but which are generally given the alphabetical symbols s, p, d, f for = 0, 1, 2, 3, respectively. The other two quantum numbers establish the number of groups within a block.

Final Form

The periodic table of the chemical elements has thus reached its final form, firmly grounded on atomic theory, and it seems unlikely that any other version will now dislodge it from its dominant position. The only unresolved difficulty is the location of hydrogen and helium at the top of the table.

Strict adherence to electron orbital theory would mean placing these elements above lithium and beryllium at the head of the s-block. Unfortunately, they have little or nothing in common with these metals except a formal oxidation state of 1 in the case of hydrogen and lithium. Alternatively, these elements can be placed to the right of the table above fluorine and neon. And while hydrogen may sit uneasily above the halogen family, helium certainly sits comfortably as a member of the noble gases. Yet hydrogen is not without some properties in common with fluorine. Both are diatomic gases, both form a variety of single bonds to other elements, both can exist as anions. The issue has been resolved by placing hydrogen and helium in a separate 1 s-block of the periodic table on the right hand side. While helium will always be found in this location, hydrogen is such that it can justify being sited at either side, or even in the centre of the table. What information should a periodic table contain?

Each box of the table must contain the essential information of the atomic number, the element’s identity (name) and its agreed chemical symbol (formula). Beyond this, the information is arbitrary, although almost without exception the relative atomic mass, that is, the atomic weight, is included as this is probably the single most important piece of information sought from the table, and it has the merit of historical continuity.

The periodic table can act as a very useful framework for classifying information, and periodic tables with data of use to chemists, physicists, spectroscopists, metallurgists and others have been produced. Some tables have up to 20 pieces of numerical data in each box.

The periodic table is, and probably always will be, the trademark of inorganic chemistry. Its beauty, its simplicity and its coding of fundamental laws of nature are unsurpassed. It has even been turned into a card game!

© John Emsley