The periodic table turns 150 years old in the year 2019, which has been appropriately designated as the International Year of the Periodic Table by the UNESCO Organization. To many scientists the periodic table serves as an occasional point of reference, one that is generally considered to be something of a closed book. Of course they, and the general public, have become aware of the ever-growing list of new elements that need to be accommodated into the table, but surely the main structure and principles of the table must be fully understood by now?
Well it turns out that this is not the case. In this blog I will touch on just some of the loose ends in the study of the periodic table. The first has to do with the sheer number of periodic tables that have appeared, either in print or on the Internet, in the 150 years that have elapsed since the Russian chemist Dmitri Mendeleev first published a mature version of the table in 1869. There have been over 1000 such tables, although some of them are best referred to by the more general term periodic system, since they come in all shapes and sizes other than table forms, with some of them being 3-D representations.
Given that there are so many periodic systems on offer it is natural to ask whether there might be one ultimate periodic system that captures the relationship between the elements most accurately. This relationship that lies at the basis of the periodic system is a rather simple one. First the atoms of all the elements are arranged in a sequence according to how many protons are present in their nuclei. This gives a sequence of 118 different atoms for the 118 different elements, according to the present count. Secondly, one considers the properties of these elements as one moves through the sequence, to reveal a remarkable phenomenon known as chemical periodicity.
It is as though the properties of chemical elements recur periodically every so often, in much the same way that the notes on a keyboard recur periodically after each octave. In the case of musical notes the recurrence can easily be appreciated by most people, but it is quite difficult to explain in what way the notes represent a recurrence. In technical terms moving up an octave on a keyboard, or any other instrument for that matter, represents a doubling in the frequency of the sound.
Octaves in the case of elements, if we can call them so, are not quite like that. There is no single property which shows a doubling each time we encounter a recurrence. Nevertheless there are some intriguing patterns that emerge among the elements that are chemically ‘similar’. For example consider the number of protons in the nuclei of the atoms of lithium atom (3), sodium (11) and potassium (19). An atom of sodium has precisely the average number of protons among the two flanking elements (3 + 19)/2 = 11. This kind of triad relationship occurs all over the periodic table. In fact the discovery of such triads among groups of three similar elements predates the discovery of the mature periodic table by about 50 years.
Now most of the periodic tables that have been proposed display such triad relationships and so we must look elsewhere in order to find an optimal table, assuming that such an object actually exists. One possible course of action might be to consult the official international governing body of chemistry or IUPAC (International Union of Pure & Applied Chemistry) to see what their recommendation might be. The IUPAC organization has a rather odd policy when it comes to the periodic table of the elements. The official position is that they do not support any particular form of the periodic table. Nevertheless in the IUPAC literature one can find many instances of a version of the periodic table that is sometimes even labeled as “IUPAC periodic table”.
And if that’s not bad enough, the version that IUPAC frequently publishes, as shown in figure 1, is rather unsatisfactory for reasons that I will now explain.
Consider the third column from the left, or as it is aptly known, group 3 of the periodic table. Unlike all other columns of the table this group appears to contain just two elements, scandium and yttrium, shown by their symbols Sc and Y. If you look closely at the numbers in the two shaded spaces below these two elements you will see a range of values, such as 57-71 in the first case. This occurs because the elements numbered from 57 to 71 inclusive are assumed to fit in-between element 56 and 72, naturally enough. The reason why the two sequences of shaded elements are shown below the main body of the main table in a mysteriously detached manner is purely pragmatic.
It’s because doing otherwise, would produce a table that is perhaps too wide as shown in figure 2. But in a sense the far-too-wide table is more correct, since it avoids any breaks in the sequence of elements and avoids the impression that the shaded elements somehow have a different status from all others or that they represent something of an afterthought. But switching to such a wide table would not solve the problem even if IUPAC were to endorse doing so. This is because the table in figure 2 still shows only two elements in group 3 of the table and because it would imply that there are 15 so-called f-orbitals in each atom, whereas quantum mechanics, that provides the underlying explanation for the periodic table, suggests that there should be 14 of them.
OK, you might say, we can easily fix the problem by tweaking the periodic table slightly to produce figure 3. As far as I can see, from a lifetime of studying and writing about the periodic table, figure 3 is precisely the optimal periodic table that IUPAC should be publishing and even endorsing officially. This table restores the notion of 14 f-orbital elements as well as removing the anomaly whereby group 3 only contained 2 elements, since it now contains four, including lutetium and lawrencium.
Why will IUPAC not see things quite so simply? That’s a big and complicated question which I can only touch upon here. Like many organizations with rules and regulations, when push comes to shove, decisions are made by committees. As a result, the science takes second place while the various committee members vie with each other and ultimately take votes on what periodic table they should publish. Unfortunately, science is not like elections for presidents or prime ministers, where voting is the appropriate channel for picking a winner. In science there is still something called the truth of the matter, which can be arrived at by weighing up all the evidence. The unfortunate situation is that IUPAC cannot yet be relied upon to inform us of the truth of the matter concerning the periodic table. In this respect there is indeed an analogy with the political realm and whether we can rely on what politicians tell us.
Featured image credit: Retro style chemical science by Geoffrey Whiteway. Public Domain via Stockvault.