Hund's Rule

User 62a37f4796

19-07-2011 17:35:56

As long as you're still working on electron objects, it would be nice to have a way to represent Hund's rule as it is taught in most undergraduate courses.


I've attached a snapshot of a ChemDraw version, since I can't figure out how to do this in MarvinSketch.


If there is a way to do this that students could manage, I'd like to know about it. If it's not there already, it would be nice to have.


Thanks.


 

ChemAxon 990acf0dec

20-07-2011 09:20:04

Hi,


With the exception of carbon you can use radicals, as follows:


Li - Monovalent


Be, O - Divalent Triplet


B, N - Trivalent Quartet


See the attached image as well.


What should be the radical state for carbon do you think?


Best regards,


Akos

User 62a37f4796

20-07-2011 12:43:10

Therein lies the problem.


By the time students are taught about Hund's rule they've never heard of radicals and lone pairs. So students would never be able to solve a problem like this with the states you mentioned.


All of these are ground-state single atoms. All (implicitly) assume an sp3 hybridization before that concept is even mentioned. This is building up to the "octet rule". Determine the number of valence electrons. Add 1 dot for every electron until all four points on the compass are taken, then start doubling up. You can easily follow Hund's rule of maximum spin multiplicity if you think of this as an arrow diagram.


Li = [1s2] 2s1 = 1 dot = Up arrow


Be = [1s2] 2s2 = 2 dots = Up arrow + Up arrow


B = [1s2] 2s2 2p1 = 3 dots = Up arrow + Up arrow + Up arrow


C = [1s2] 2s2 2p2 = 4 dots = Up arrow + Up arrow + Up arrow + Up arrow


N = [1s2] 2s2 2p3 = 5 dots: 1 paired & 3 unpaired = Up/Down arrow pair + Up arrow + Up arrow + Up arrow


O = [1s2] 2s2 2p4 = 6 dots: 2 paired & 2 unpaired = Up/Down arrow pair + Up/Down arrow pair + Up arrow + Up arrow


 


I think you get the pattern by now. The point is to make students think about (valence) electrons in the first place, and by extension what those unpaired electrons mean in terms of their ability to form bonds: Li can make 1 bond, Be 2 bonds, B 3 bonds, C 4 bonds, N 3 bonds, O 2 bonds, F 1 bond, and Ne has none left so no bonds ..


 


You obviously already have the graphical capabilities to do it. Making
it available to faculty and students in a knowledge-sensitive fashion
would be great. For instance, if there were a menu item akin to "Switch Radical" that would allow a student to push up to 8 electrons onto an atom following the pattern described here and the images attached previously, that would really please my faculty members.


 


Thanks.

ChemAxon 990acf0dec

20-07-2011 14:08:35

Hi,


OK, I understand it now. The problem is that Marvin shoudl follow real chemistry as far as possible, so any feature like this should be hidden by default, and be available only by customization. If I understand well, you refer to the "Switch Radical" button in this example:


http://www.chemaxon.com/devel/marvin/examples/applets/sketch/studentexam/index.html


but I think you actually need a button that rather behaves like the "Increase Lone Pairs" button, creating single electrons, upt ot 4, and then pairing them up to 8.


We will consider this request when planning the next releases.


Best regards,


Akos

User 62a37f4796

21-07-2011 12:12:34

Exactly. Not just increase, though. Decrease should be possible too. I don't know how many time I've overshot my target and needed to "go one down" with lone pairs.


 


What you mean with "real chemistry" is debatable. (Do you consider the current implementation with 2 lone pairs on a carbon atom to be "real"? Shouldn't that be 1 pair in 2s2 and 2 unpaired electrons divided between 2px and 2py?) Electrons do not take up fixed positions around an atom. Neither do they travel in pairs. So the whole formalism isn't "real" in any sense of the word, though it's mighty helpful to explain things.


Going over the use-cases I've seen, we don't just have the "octet rule".


1st row elements: "duet rule" .. 1s orbital only allows for Up and Up/Down


2nd row elements: "octet rule" .. sp3 hybridization assumed for all so 4 energy levels and 8 electrons, hence "octet"


3rd row and higher main group elements: "expanded octet rule" .. sp3d2 hybridization assumed possible for all so 6 energy levels and 12 electrons


transition metals: "18 electron rule" .. we never even cover this in the context of electron configuration/valence electrons/Hund's rule since there are too many exceptions


All in all, I think I've only seen Hund's rule applied in the GenChem curriculum for the duet/octet situation. If we can cover those, that'd be great. I don't want to preclude a more comprehensive solution, though, if someone has a better idea.


 


Thanks again.

ChemAxon f052bdfe3c

28-07-2011 12:21:06

Thank you for clarifying the problem.  I agree, the “expanded octet rule ” and the “18
electron rule ” would be extremely hard to handle; the “duet” and “octet” rules
can be visualized more easily.  As Akos
wrote:  we would
consider it as  a featured improvement when
the next releases is planned.  


Best regard,


Efi

User 62a37f4796

06-12-2012 14:17:31

Thanks for implementing this new feature (initially in 5.8?). It will greatly help our students and faculty introducing the topic of Lewis structures, Hund's rule, and the duet/octet rules.


When I'm looking at student submitted answers, I always check the CXSMILES/CXSMARTS first before I apply an XML parser.


I noticed that in the raw MRV/XML the new features are lumped in with the radical group: radical="trivalent4 4", but I don't see that reflected in the CXSMILES/CXSMARTS: ^1 and ^2 are returned, but not ^3 and ^4. Is there a reason to (not) do this?


I've attached a screenshot of the first 3 period elements with their Lewis graphs, the MRV and the CXSMILES/CXSMARTS generated with 5.11. Note how the latter two do not capture all the information in the former.


Can this be added in the future?


Thanks,


   Michiel

ChemAxon f052bdfe3c

13-12-2012 20:49:28

We forwarded this issue to our Core team.


Best Regards,


Efi