Conversations, Confessions, Confusions (and hopefully some Clarity) on Electronic Configurations

I have a confession to make: I don’t really understand how to correctly predict the electronic configurations (EC) of every element and ion. 

Until recently, I thought I had everything figured out.  In fact, for quite a long time I have believed that I knew – at least in principle – how to predict the EC of any element or ion. Having read several articles in the Journal of Chemical Education on this subject,^1,2 I understood that the ground state EC of an atom or ion is a function of the lowest energy arrangement of ALL electrons in that atom or ion. I understood that confusion arises in predicting EC when the focus is placed solely on the energy of the very last electron (or last few electrons) being “placed in” orbitals during the “building up” of the atom or ion.

But I also held a misconception regarding EC: I thought that the 4s orbital was ALWAYS higher in energy than the 3d orbital. My justification for this erroneous belief came from information in two papers published in JCE^1,2. I won’t bog you down with all the details, but keep in mind that the title of one of these papers is “4s is Always Above 3d!”¹ However, I learned that this idea is incorrect during a conversation on Twitter with Dave Doherty. Dave alerted me to another publication in JCE which claims that in K and Ca, the 4s orbital is lower in energy than the 3d!³ After reflecting on that conversation with Dave, the arguments in these various JCE publications, and the data contained within, I concluded that my understanding was wrong. 4s is NOT always above 3d!

What is my point in sharing all of this with you? Well, consider that I have been teaching science and chemistry for over 20 years, I have a Ph.D. in physical chemistry, and I still don’t completely understand what is going on with EC. It is my strong suspicion that other teachers of science and chemistry also struggle with completely understanding this material and explaining it to students. There appears to be no way to present the concept of EC to students in a manner that is easy to grasp and faithful to physical reality at the same time.  Consider that one author has argued that five concepts are needed to understand “The Full Story of the Electronic Configurations of the Transition Elements”³: d-orbital collapse, d vs. s electron repulsions, s-Rydberg destabilization, configurations and states in free and bound atoms, and relativistic spin-orbit coupling. I don’t know about you, but these concepts are going to be tough for my students to understand – and present challenges to my intellectual horsepower, too! Eric Scerri has written extensively on the electronic configurations of the elements, and he has noted that “contrary to what some educators may wish for, there is no simple qualitative rule of thumb that can cope with this complicated situation”.⁴

I shared with Dave my frustrations on this subject.  I communicated with him my desire to present the prediction of the EC of elements and ions to my students in a manner that:

1. Is understandable to both teachers and students alike.

2. Chemical educators can generally agree upon.

3. Is faithful to physical reality as much as possible.

4. Can be justified with theory that both teachers and students can understand.

Dave met my challenge head on, and put together some ideas on how to present a coherent and accurate picture of electron configuration. I encourage you to read his article, Clarifying Electron Configurations! 

Dave Doherty develops physically accurate, 3D particle models of atomic and molecular structure, chemical reactions, and other chemical concepts and the Atomsmith software⁵ that allows students and teachers to interact with and “perform experiments” on these models. He can be reached at ddoherty@bitwixt.com. He uses @Atomsmith1 on Twitter to share ideas about chemistry education and he uses @AtomNMolecules to challenge college chemistry professors on topics such as electron configuration.

References:

1. Pilar, F. L. 4s is Always Above 3d!, J. Chem. Educ., 1978, 55, 2 – 6.

2. Reed, J. L. The Genius of Slater’s Rules, J. Chem. Educ., 1999, 76, 802 – 804.

3.  Schwarz, W. H. E. The Full Story of the Electron Configurations of the Transition Elements, J. Chem. Educ., 2010, 87, 444 – 448.

4. Scerri, E. The trouble with the aufbau principle, 2013, http://www.rsc.org/eic/2013/11/aufbau-electron-configuration.

5. Atomsmith Classroom, Bitwixt Software Systems, www.bitwixt.com. Available for Mac and Windows computers, and as an online HTML5 app for browsers on all platforms.