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Book Review: The Periodic Table: Its Story and Its Significance

PTOEQuick! 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.

By July 13, 2009 9 comments Uncategorized

Alternative Periodic Tables (Updated. Now with a Final Thought!)

There’s an article in the current issue of Nature Chemistry that discusses some alternative ways of depicting the periodicity of the elements. There’s the IUPAC recognized Mendeleevian periodic table that everyone knows. There’s probably 3 in eyesight of you right now, isn’t there?
mendeleev(click for larger)

d

d

d

But how many alternative periodic tables can you think of.  Hint: there’s A LOT.  More below the jump.

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By April 26, 2009 88 comments general chemistry