Book Review: The Periodic Table: Its Story and Its Significance
by azmanam on Jul 13 2009 (3960 Views)
Quick! Jot down the definition of an element. OK, now imagine you don’t know what an atom is. The proton hasn’t been discovered yet. Isotope isn’t even a word. Now what do you do? These were the issues with which the early chemists were struggling as the handful of known elements was being classified and organized. Eric Scerri has chronicled this history nicely in his book The Periodic Table: Its Story and Its Significance (2006, Oxford University Press).
I don’t consider myself a history buff, but I really enjoy reading historical accounts of the development of our esteemed field. The story presented in The Periodic Table was fascinating and engaging throughout. Scerri (UCLA, editor Foundations of Chemistry) couched the history of the periodic table in the larger history of the discovery of the elements themselves. How did new elements fit in with the current understanding of the periodic table? What happened when the task became more difficult with the discovery of the noble gases – a family which no one predicted and which threatened to destroy the periodic system entirely?
The book had one official theme, and one unofficial theme. First, can chemistry be explained by quantum mechanics; that is, can chemistry be reduced to physics? Until the 1920s, the answer was definitely no. It wasn’t even possible to construct the necessary equations once more than one electron was considered. Scerri elegantly walks through a brief history of the evolution quantum mechanics as the math was invented to recognize the electron as both a wave and a particle. With the new quantum mechanics, the quantization of the angular momentum of the electron is deduced from first principles for the first time.
However, Scerri also points out how various aspects of chemistry still cannot be derived from first principles. For example, the Aufbau principle is experimentally known, but not derived independently. The same is true for exceptions to Hund’s rule. Several transition metals accept a less than full s orbital in order to add an extra electron to the d orbital. Ab initio and the density functional approach have advanced quantum mechanics significantly, yet when the various failings are considered, Scerri notes that answer to the reductionist question is both ‘yes’ and ‘no.’
The theme Scerri spent the most time discussing concerns the nature of an element. Are elements simple substances or basic substances? And as a subtheme, how (if at all) do elements survive when combined in compounds? Scerri could have done a better job defining the terms, as I didn’t have a firm grasp on this issue until I was reviewing my notes for this review. I was confused throughout the book in this matter. I think I finally get it.
The example given in the book involves the halogens. Physically, fluorine and chlorine are gases, but bromine is a liquid, and iodine a solid. Yet, chemically, they all form white solids when combined with elemental sodium. Furthermore, when combined, neither the physical properties of elemental sodium nor the physical properties of elemental chlorine are manifest. So there is a disconnect between the observable properties of an element (elements as simple substances) and the abstract, or chemical, properties of an element (elements as basic substances).
Different chemists had passionately held convictions on the matter, and it was fun to watch Scerri illustrate the disagreements throughout time. Lavoisier was in the simple substance camp, Dalton and Mendeleev sided with the basic substance chemists. It was interesting to watch these philosophies compete with each other throughout the book. Scerri argues for the classification of elements as purely basic substances; however, his argument failed to resonate with me because I couldn’t keep the terms straight. At times it felt like Scerri was arguing against it and for it on the same page. I understand his point and agree that abstract properties (valence electrons, for example) are better descriptors of elements than physical properties, but his argument fails to convince me to rearrange the periodic table to put, for instance, helium above beryllium because both have two valence electrons.
As might have been expected from the comments to the previous post, the issue of atomic triads appeared throughout the book. It was fascinating to read about them as they were discovered. They were integral to initial discovery of the periodicity of the elements. Chemists noticed that for certain groups of three chemically similar elements, the atomic weight of the middle element was approximately the average of the outer two elements.
Yet the theory was fundamentally flawed. The periodic table is no longer organized fundamentally by atomic weight, but by atomic number. While it is true that organization by atomic number actually makes more triads work, it also shows triads as merely a necessary coincidence given what we know about orbitals and how they’re filled. Elemental triads only show that the middle element is equidistant between the two outer most elements. Because the shells fill in order 2,8,8,18,18,32,32, the elements in the every other row are necessarily triads. That is, all elements 3-10, 19-36, and 72-86 (correction: 9-12, 31-38, 71-80) are necessarily the middle of a triad. It was important and interesting to see how they influenced the ability of chemists to develop the periodic law, but I don’t feel they have much more meaning today than a statistical coincidence of some groups of elements within a family.
Overall, the book was a very interesting read. It’s nice to step out of the trees for a while and see the forest. Chemistry is a pretty amazing field, and we constantly strive to push the boundaries of the status quo. The only real way to get ahead, though, is to know where you came from. I recommend Eric Scerri’s The Periodic Table for everyone interested in some of the stories of the origin of our profession.
Thanks for a very fair review of my book.
A couple of responses if I may. Talk of electrons and configurations does not amount to talk of elements as basic substances. Basic substance is an abstract concept.
I no longer support the left-step table as I did in the conclusion of this book. I now favor a table with H in the halogens and He remaining in the noble gases. There have been articles about this in Journal of Chemical Education and American Scienntist.
Finally, on the question of triads, I think that it may be more than a mere numerical conicidence. More recentlty I have argued that the nucleus has a causal role to play in determining electronic configurations and that the nucleus and relations involving numbers of protons, such as atomic number triads, may play a more fundamental role in the placement of elements in the table than just electronic configuration.
Consider this. In the placement of hydrogen, in the conventional table, we consider the number of outer shell electrons that are present. But in the placement of He we end up by considering the number of electrons needed to fill the outer shell, namely zero, in placing He in the noble gases.
This is inconsistent. If we considered the number of electrons needed in H it would end up in the halogens, which is actually what I am recommending these days.
Bottom line, electronic configuration leaves group membership under-determined. The possible more fundamental way of deciding on group membership is the formation of perfect atomic number triads. If this is granted, H fits into the halogens, He remains in the noble gases and group 3 consists of Sc, Y, Lu, Lr and not Sc, Y, La, Ac as many books and websites continue to show.
I will be away in Colombia for 10 days and may not have access to the Internet but hope to resume these discussions on my return. Thanks again for the thoughtful book review azmanam.
regards,
eric scerri
This is where I have the most trouble wrapping my head around simple vs basic substances. You mention electronic configuration does not equate to discussing basic substances. Ok, I can understand that. You also mention in your book Mendeleev’s rejection of elements as simple substances, and how his ability not to be ‘misled by nonessential chemical properties’ perhaps helped him to construct his periodic table better than the other guys. Then you suggest reorganizing the periodic table based on electronic configurations – but only insofar as they create a new completed triad? I get so lost keeping the terms straight that I can’t fully appreciate the theory for what it is.
A little more clarification on the question of element as simple substance and as element as basic substance.
There are actually three senses of the term element to consider. The third one is unfortunately often confused with element as basic substance.
The third sense can be called “combined element”.
Element as basic substance is the abstract entity which underlies both element as simple substance AND the combined element.
For example, there is sodium which is in the form of a soft gray metal (simple substance) and there is the sodium that is present in NaCl (sodium as combined element). The element as basic substance is the name given to what these two forms of sodium have in common. If one insists on a microscopic interpretation it would have to be just the nucleus with 11 protons and the appropriate number of neutrons.
I hope this helps.
The current issue of Foundations of Chemistry contains two interesting articles which explore the concept of ‘element’ in great depth by two philosophers of chemistry.
Moreover if anyone is interested in dropping in, The annual conference for the International Society for the Philosophy of Chemistry will be taking place at the Chemical Heritage Foundation, in Philadelphia, starting on Th, Aug 13 up to Aug 15th. The society has a website if you search for ISPC. The program for the conference can be found at the website for Chemical Heritage Foundation.
At least two papers will be on the concept of element.
regards,
eric scerri
And how do new elements like rutherfordium fall within the triads?
Rutherfordium becomes the third element in a triad with zirconium-hafnium-rutherfordium. In fact, all the elements 103-112 (now known as copernicium).
Visually, here are the positions of the triads in the periodic table. The blue shaded elements are the middle element of a three-element triad. Thus, fluorine is the first element shaded blue because it is the middle of the hydrogen-fluorine-chlorine triad.
colored triads
The pink elements are elements that will be the middle element of a triad… once the elements underneath them are discovered and recognized. The elements with red shading over the atomic number are elements that have been moved to allow for more completed triads.
When you rearrange the periodic table even further, the triads begin to form a nice pattern: long triads. It’s easy to see how such a layout is tempting when you subscribe to the triad school of elemental ordering.
Thanks for illustrating the triad concept so nicely.
May I make a further suggestion.
If you use a left-step periodic table the triads become clearer.
The conventional medium-long form table confuses the issue slightly.
I hope you will take up this suggestion and make a further illustration.
eric scerri
sorry I see you have already done it!
under “long triads”
thanks
eric scerri
scerri is a pervert. He should be fired.
very funny! And the joke about “still alive” is rather sick and perhaps even perverted!
Scerri