[A level Chem] Max OS of Transition Metals
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There is a "rule of thumb" where the max oxidation state of a 3rd period transition element could be estimated by the number of 4s electrons + unpaired 3d electrons
eg.
Max OS of Mn [Ar] 3d5 4s2 = 2 + 5 = +7
Max OS of Fe [Ar] 3d6 4s2 = 2 + 6 = +6
Could someone shed more light on how the paired/unpaired 3d electrons is a factor in this context?
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Originally posted by atomos:
There is a "rule of thumb" where the max oxidation state of a 3rd period transition element could be estimated by the number of 4s electrons + unpaired 3d electrons
eg.
Max OS of Mn [Ar] 3d5 4s2 = 2 + 5 = +7
Max OS of Fe [Ar] 3d6 4s2 = 2 + 6 = +6
Could someone shed more light on how the paired/unpaired 3d electrons is a factor in this context?
Heh, quite a hornet's nest being stirred up here.This is rather beyond the A level syllabus, so do let your students know that there's no need to worry over it. In fact, for the Singapore H2 syllabus, H2 Chem students are not even required to show knowledge of the "rule of thumb" that you mentioned, which is required only for some of the other A level syllabuses around the world (even these other A level syllabuses only required the students to be aware of the rule, but not require students to explain it).
There is more than one way to look at this, or to understand and explain this, but let's keep it simple here (ie. other explanations require more in-depth Uni level concepts, not to mention the existence of multiple anomalies, eg. Cu, that makes inorganic chemistry so thoroughly entertaining, if you will).
I'll give two simplified reasons which may help the student to understand whyfore this "rule of thumb", but these reasons are by no means exhaustive, and the keen student will no doubt have many further questions, but it would be more appropriate to deal with these at the University levels (rather than A levels).
1st reason : in some ways, pairing of electrons have a 'stabilizing' effect (although in some other ways, the inter-electron repulsion of paired electrons have an opposite 'destabilizing' effect; I already warned you that these questions would beget further questions, quite beyond the ken of the A levels), in the sense that (similar to the concept of free radicals being extra unstable and reactive) the unpaired d electrons are more available and willing to be donated away for ionization (in the case of lower OSes) or for occupation of covalent bonding orbitals with other elements (usually electronegative O atoms), in the case of higher OSes.
2nd reason : coincidentally (so to speak), it so happens that simultaneously, the increasing effective nuclear charge, electronegativity and ionization enthalpies (as we go from left to right of the d block elements) reaches the 'tipping point' after Manganese, and as such increasingly prohibits these elements from losing electrons and exhibiting higher OSes.
As I mentioned previously, these reasons described above are certainly a simplification (of the wonderous nuances and exponentially increasing variances of inorganic chemistry, particularly transition metal chemistry), but will more than suffice for the A level student (and certainly so for the Singapore H2 Chemistry student).
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Thanks for the help!
Yes, I was trying to link it to the A level concepts and could only come up with something similar to the two points you brought up.
But I was wary of oversimplifying or worse, using wrong concepts.
Then it got more tedious when I tried to picture in obitals sharing during covalent bonds.
Good thing is, like you said, that the "rule of thumb" serves little propose in actual answering of A level questions.
Anyway, if you do have links to any further reading up on this, I be happy to give it a go.
