[aggsquirrel]

Hello all,

I am taking an undergraduate introduction to inorganic chemistry course. I have had a little difficulty coming up with a sufficient explanation for a problem. To summarize the question, why does krypton only form krypton difluoride and its derivatives, only gaining an oxidation state of +2, while Xenon has many fluoride compounds and can have an oxidation state exceeding +2? I have done a little poking around here and couldn't really find anything that was specific to this question since it seems like krypton chemistry is not really a large field, the reason for this is also likely the answer to my question. In my text, housecraft and sharpe,a later chapter on group 18 chemistry alludes to krypton chemistry stating that the reason why only KrF2 is stable has to do with molecular orbital theory. I have done a bunch of reading both in the text and out of it and can't really come to a conclusion to my problem. Any help on this would be much appreciated, since I have been pulling my hair out musing on this for the last day or two. Thanks in advance!

[AWK]

KrF4 is known from 1963.
Science 15 March 1963:
Vol. 139 no. 3559 pp. 1047-1048

[aggsquirrel]

KrF4 is known from 1963.
Science 15 March 1963:
Vol. 139 no. 3559 pp. 1047-1048

Thank you for the article! Unfortunately my institution's subscription has been suspended. I contacted the librarian about this so I can read the full article. I was however able to read the first page and from what I can gather, the tetrafluoride is far less thermally stable than the difluoride variant. Could I be looking at this problem wrong? Could this merely be a case of sterics? In order for the tetrafluoride to exists it must adopt a 6-coordinate geometry, octahedral which would place the fluorines planar to one another and the two lone pairs in the orthogonal positions. This would be less hindered for a larger Xe atom and even more so for a Rn atom. Conversely the difluoride is able to exist more stably because the two fluorines can occupy the two linear positions in a trigonal bipyramidal geometry while the three lone pairs are able to take the planar positions. Maybe I have been confusing myself trying to apply some MO theory to this, thinking about drawing from the lower energy s and p orbitals or d orbital depending on if you believe a molecular orbital can show d character.

[fledarmus]

Is it possible that this is a size phenomenon? Keeping in mind that inner shells of electrons essentially shield the nuclear charge from the outer shells, the outer shell electrons in Kr and Xe will both see approximately the same nuclear charge (number of protons - number of inner shell electrons), but the distances between those electrons and the nucleus are quite different. If I remember my physics correctly, the attractive force between charged particles is a function of the square of the distance, so the attractive force between a Kr nucleus and its outer shell electrons will be higher than the the attractive force between a Xe nucleus and its outer shell electrons. Since both are reacting with the same atom (F), the attractive force between the F and the electrons are the same in both cases. It seems logical then that it would be easier to pull more electrons from a Xe atom with F than from a Kr atom.