Recently, the BBC reported that “a periodic table found during a laboratory clear-out at the University of St Andrews is believed to be the oldest in the world.” Here it is, together with the link to the original article.
There’s loads of interesting and thought-provoking science in this periodic table. Hence this post. It would be helpful to know a bit about the ‘modern’ periodic table – you can find the first part of our series on atomic structure and the periodic table here. And you don’t need any German. The title is approximately ‘Periodic regularities of the elements, after Mendeleev’, and it won’t take all your problem solving skills to translate ‘Gruppe’ into ‘Group’.
So where to begin? Well, the first point is that this periodic table dates from 1885, and represents the state of knowledge at that time.
General patterns
The table shows mass numbers, but no atomic numbers. The elements are ordered by mass. No-one is yet counting protons in atoms (which is what the atomic number is about). Because nobody knows about protons yet.
The elements are organised into ‘groups’ (columns), but not according to atomic structure, such as the number of protons or the configuration of electrons. After all, what is atomic structure in the 1880s? The concept of atoms was at that time quite fiercely debated – were they real, or just a theoretical counting mechanism? If they were real, they certainly weren’t thought to have any ‘structure’. Instead the groups are devised according to their chemical behaviour, and specifically what oxides and compounds with hydrogen they form (look in the column headings of the table to see this). Just think about the amount of theoretical and practical knowledge and experience that goes into isolating all these elements, and being able to determine the oxides and hydrogen compounds that they do form!
There are about 70 elements in the table. Dozens are yet to be discovered! And for those of you who think you could do better, here’s a quiz from Sporcle on naming as many elements as you can…
It is part of the lore of chemical history that Mendeleev left gaps in his periodic table, expecting elements to exist to complete the pattern. Then as those elements were discovered with the properties he predicted, his periodic table gained credence. And in the table above, we see gaps at 72 and 165 among others. (Either that, or the labels have fallen off after nearly a century and a half).
Some specifics
There are no ‘noble gases’ in the table. Noble gases, also called ‘inert gases’ are those that are placed in the right-most group of a modern periodic table – they include helium, argon, neon, krypton and xenon. Their atomic structure is particularly stable, which means that they are very unreactive – they chemically don’t do very much. Things tend to be hard to find when they don’t do much (Similarly, in the world of particle physics (link) the neutrino was postulated in the 1930s and it was decades before it was discovered).
And so by 1885, none of the noble gas elements had been discovered. Even argon, which constitutes 1 % of the Earth’s atmosphere, was discovered as late as 1894. And helium, which is rare on earth, but makes up 25 % of the visible universe. (Indeed, helium was discovered in the Sun through spectroscopic analysis of sunlight before it was found on earth – I once had a 1912 textbook in which helium was called coronium, after the corona of the Sun).
There may be no noble gases, but a Group 8 has still been constructed. It contains many of what we would call the transition metals. This is interesting, because the transition metals have complicated and various oxidation states. This makes their valencies (the number of bonds the atoms form) tricky to predict. And since those oxidation states/valencies determine the compounds they form, such as the oxides and hydrogen compounds upon which the table is based, it must have been difficult to find places for them in the table.
What’s ‘J’ at 127? I’d never heard of this. Well, apparently, before iodine was called iodine, it was called jodium!
And what’s ‘Di’ at 145, with a question mark?! I’d never heard of this either. It turns out that Di stands for didymium, which is a mixture of the elements praseodymium and neodymium. This speaks volumes about the difficulty of isolating elements. How do you know when you have a pure sample of an element? I have a feeling we take for granted the separation/isolation process. But when you look at a picture of a block of zinc (say), how do you know it’s pure zinc? In the case of Di, it can’t have been obvious at all, as the state of the chemical art still placed it as an element in 1885! The mixture didymium has modern day uses, such as colouring glass and in the manufacture of safety glasses.
I always wonder how those who have to teach the ‘history of the periodic table’ make it engaging – after all, many students have enough trouble with the ‘normal’ one. I hope this discovery at St Andrews helps!