Louis: Dr. Scerri, it’s a privilege to speak with you; thank you so much! You are an expert in the periodic table and in the history of its discovery. Why did you choose this topic in the history of science? What intrigued you about it?

Dr. Scerri: One of the many things that I’ve always liked is classification systems of any kind. The periodic table seems to me to be the ultimate classification system because it deals with the materials that everything is made out of. I also like it because it unifies everything and I’m interested in unification. I have always liked the periodic table ever since I was a kid. The idea is that all the knowledge about the elements is contained within it, so you can have knowledge about the way electrons are distributed in atoms, and knowledge about metals and nonmetals, which is much more microscopic, much more large-scaled, and it’s all there, all in that one chart! So there are several reasons, not one.

Louis: In your article “The Evolution of the Periodic System” you introduce the history of the table as being 200 years old. What event marks the beginning of that history (and why)?  

Dr. Scerri: I would not have chosen to say 200. This was done by the editors of the magazine!

Louis: So, when do you mark the beginning of the history of the table? Would it be 1787, when Antoine Lavoisier and his colleagues created a list of 33 elements, which they knew back then (as you mention in this same article)?

Dr. Scerri: Not really. A periodic table implies a two-dimensional representation. Lavoisier’s was just a list and so just one-dimensional. De Chancourtois, a French engineer, published the first true periodic system in 1862.

Louis: Dimitrii Mendeleev is considered to be the key person in the discovery of the periodic law. However, during the time when Mendeleev made his discovery, a number of other scientists made similar discoveries partly as a response to the discussions of the famous Karlsruhe Conference in 1860. Why this Conference?

Dr. Scerri: The significance of this Conference was that it was the first time that scientists agreed about the atomic weights of most elements. This was an essential pre-requisite to ordering the elements, which is itself the first step towards producing a periodic table. 

Louis: Thus, the Conference was a way for scientists to communicate what they doing individually in their home laboratories, and to move forward with development of periodic law as a community?

Dr. Scerri: Yes more or less. Except not necessarily in home laboratories. By this time, the 1860s, people were already working collectively in universities.

Louis: What was so unique about Mendeleev’s approach to periodicity, which distinguishes him from other scientists?

Dr. Scerri: Well the obvious answer to that is Mendeleev made predictions about yet to be discovered elements; and those predictions came true, at least in three famous cases. These three cases were discoveries of the elements eventually called gallium, germanium and scandium after their discovery. But a qualifier about that is that Mendeleev made a total of eighteen predictions, of which only nine were actually discovered. Several elements that Mendeleev believed existed, such as newtonium, coronium and ether, never actually materialized. Thus, the traditional answer will be, like most experts have concluded, Mendeleev is the leading discoverer because he made predictions. Others didn’t make predictions. If they did, as in the cases of John Newlands and Lothar Meyer, they were quite timid about it, didn’t go into any details, and the predictions didn’t play out. A more a general answer would be that Mendeleev made the periodic table his own thing. The others may have discussed the periodic table, they may have published a paper or two, but they didn’t pursue it to the extent that Mendeleev did. For Mendeleev, the periodic table meant a tremendous amount: he put a lot of energy into it. So he is rightly considered the main discoverer, although, as you mentioned, he wasn’t the only one. In fact, he is the last of the six discoverers of the periodic system. The last but the best.

Louis: Just to add a question about Mendeleev’s predictions, would you say that predictions really matter in science? Are they really important?

Dr. Scerri: Sure. Because it is all very well for a theory to explain something we already know, something that has already been observed. But there is always a suspicion that the scientist has designed his theory, has cobbled together the theory, has rigged the theory in order to come out with the real answer. It is a little bit like a student trying to solve a problem and when he (or she) gives up and goes to the back of the book and finds the answer. It certainly becomes easier because that student knows what he or she is driving at. So when you are not making a genuine prediction, you know what you are driving at. That’s called accommodation of data, as opposed to genuine prediction when nobody knows the answer in advance and it comes out right. That signifies that the scientist really has a powerful theory because it’s almost like magic: the scientist predicting the future, in a sense. It is not literally magic of course, but it gives you a sense that the theory is a deep one, which really captures something about nature. But having said that, there is an ongoing debate. In my book “The Periodic Table: Its Story and its Significance” I discuss the fact that some people believe that accommodation is just as worthy as a genuine prediction. But certainly, broadly speaking, yes, prediction is what you want from a theory.

Louis: Some scholars see your interpretation of Mendeleev’s discovery as more of a trend, observable among elements, rather than a law in a strictly scientific sense. Is this a correct vision of your explanation of Mendeleev’s discovery?

Dr. Scerri: Mendeleev discovered a law. Chemical laws are a little bit different from laws in physics in the sense that laws in physics tend to be more exact, for example Newton’s law of gravitation or Hook’s law or Boyle’s law. The periodic law is not as exact because what it is claiming is that after certain intervals there is an approximate repetition of the same element. It’s not claiming that you get an exact repetition with let’s say, lithium, sodium, and potassium. Those three elements are not identical. They are, roughly speaking, the same. So that suggests that chemical laws are kind of looser, which might then imply that biological laws might be even less “law-like” in the traditional sense.

Louis: So, the periodic law way of capturing patterns in chemical properties of elements does not represent the natural world in the same precise way as, for example Newton’s law of gravitation, right? But why? Is it because when we try to capture how the world works from a position of chemistry the world becomes more complex and not reducible to simple “elegant” formulas of physics?

Dr. Scerri: Yes. As one moves from physics through chemistry, biology and eventually sociology, the systems become more complex and cease to conform to exact laws.

Louis: What about laws in history?

Dr. Scerri: I personally don’t think you are ever going to find laws in history. I’m not sure if many people claim that there are historical laws and I don’t think there will ever be historical laws. It’s just too complicated. The systems are too complex to expect there to be law-like regularities.

Louis: So that means that tools we use to make sense of the world around us – I mean our theories – do not allow us to capture that world in all its complexity?

Dr. Scerri: Yes theories are always an approximation, never an exact replica of the way the world is.

Louis: What is the significance of how the information in the periodic table is organized – what is the structure and visual strength of the periodic table?

Dr. Scerri: If by “significant” you mean “what’s the benefit”, it allows a person to make predictions about elements, maybe about elements that have not yet been discovered even though they have all, pretty much, been discovered now. Nature forces upon us that periodic table. Of course there are various forms of the periodic table, there’s the eighteen-column, medium-long form you find in most textbooks, there’s the expanded thirty-two-column table. But in all of those cases it’s nature, it’s the real world that dictates what it’s like.

Louis: To conclude our interview, it would be outstanding if you could share with us a story of the 117^thelement which joined the periodic table recently, as you write in your 2013 article?

Dr. Scerri: Element 117 is the most recently discovered element which completes the seventh row of the periodic table and the first time that the periodic table has been complete in this way since it was first discovered. So far only fifteen atoms of this element, that will soon be given an official name, have ever been observed, of which six were observed in 2010, seven in 2012, and two in 2014. The first such discovery was announced in Dubna, Russia, by a Russian–American collaboration. Two isotopes of the element, namely ²⁹⁴Uus and ²⁹³Uus have been synthesized with half-lives of 15 and 22 milliseconds respectively.

Louis: Thank you, Dr. Scerri! It was wonderful to learn from your work and our discussion!

To learn more about Dr. E. Scrri’s work, please visit his website, www.ericscerri.com and Wikipedia entry, https://en.wikipedia.org/wiki/Eric_Scerri.