From chemistry-request- at -server.ccl.net Thu Oct 18 23:03:45 2001 Received: from carbon.chem.ucla.edu ([128.97.35.55]) by server.ccl.net (8.11.6/8.11.0) with ESMTP id f9J33jB09573 for ; Thu, 18 Oct 2001 23:03:45 -0400 Received: from Laurence.mbi.ucla.edu (pc-ll.chem.ucla.edu [128.97.35.245]) by carbon.chem.ucla.edu (8.11.4/8.11.4) with ESMTP id f9J33iq16652 for ; Thu, 18 Oct 2001 20:03:44 -0700 (PDT) Message-Id: <5.1.0.14.2.20011018200127.039acd68;at;127.0.0.1> X-Sender: lavelle/mbi.ucla.edu[ AT ]127.0.0.1 X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Thu, 18 Oct 2001 20:06:54 -0700 To: CCL From: Laurence Lavelle Subject: Summary: Orbital energies for 1st row transition metals. Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; format=flowed My original emails are first. Responses follow. Thanks to all. Laurence Lavelle >Date: Thu, 18 Oct 2001 13:53:16 -0700 >To: CCL >From: Laurence Lavelle >Subject: Orbital energies for 1st row transition metals. > >I'm still getting a number of responses (thanks), but before I get anymore >reprimands for covering this topic in a first year course let me clarify. >While discussing the ground state for Cr [Ar]3d^5 4s^1 and >Cr^+1 [Ar]3d^5 there was general discussion about the relative filling >of orbitals for ground state anions. In the past I have not discussed >electronic configurations for negatively charged transition metals, hence >my question below. > >So far I don't yet have a quantitative answer. My qualitative answer is >that the orbital occupancy will depend on the relative energy difference >between the degenerate 3d and the 4s orbitals and the spin pairing energy. > >Discussion welcome, >Laurence Lavelle > > >>Date: Tue, 16 Oct 2001 18:08:26 -0700 >>To: CCL >>From: Laurence Lavelle >>Subject: Orbital energies for 1st row transition metals. >> >>The following question relates to a first year chemistry course that I teach. >> >>Cr ground state is [Ar]3d^5 4s^1 >> >>What would be the ground state for Cr^-1 ? >>Explain the e- configuration and estimate the relative energy difference >>between Cr and Cr^-1. >> >>Discussion and comments welcome. >> >>Best, >>Laurence Lavelle >> >> >To: Laurence Lavelle >Subject: Re: CCL:Orbital energies for 1st row transition metals. >X-mailer: FoxMail 3.0 beta 1 [cn] > >I think the ground state should be [Ar]3d^5 4s^2 for Cr^-1. As we know, >3d^5 is half-filled and the energy will be lower. >To: Laurence Lavelle >Subject: Re: CCL:Orbital energies for 1st row transition metals. > >This is an interesting question, but I would not give it to first year >students. Already the ground state of neutral Cr can not be explained >using the orbital approximation. A numerical treatment including electron >correlation is needed to account for it, since the total energy of the >atom is relevant not the sum of orbital energies. What kind of answer do >you expect from first year students? That the electron enters 4s because >of greater penetration of s- compared to d-electrons? I guess that >everything depends on electron correlation again. I don't think that >subtle details can be estimated easily and I would not ask first year >students to solve problems they can not possibly answer. >To: Laurence Lavelle >Subject: Re: CCL:Orbital energies for 1st row transition metals. > >Hello, > >For (french) first year students, the only answer is : >due to Hund's rule, I would propose that the electronic >configuration of Cr{-1} anion is [Ar]3d^5 4s^2. >First year students just know Slater rules to estimate >the electronic energy. >Thus: > >E(Cr)= E(Ar) + 5*E(3d) + E(4s Cr) >E(Cr-) = E(Ar) + 5*E(3d) + 2*E(4s Cr-) > >E(Cr-) - E(Cr) = 2*E(4s Cr-) - E(4s Cr) > >E(4s Cr) = -0.5*(2.95*2.95)/(3.8*3.8) u.a. >E(4s Cr-) = -0.5*(2.6*2.6)/(3.8*3.8) u.a > >where 3.8 is the value of n* for n=4 >2.95 is the value of Z* for the 4s electron of Cr >2.6 is the value of Z* for one 4s electron of Cr- > >Z*(4s Cr) = Z - 2*1 - 8*1 - 13*0.85 = 2.95 >Z*(4s Cr-) = Z - 2*1 - 8*1 - 13*0.85 -0.35 = 2.6 > >because the electrons are put together in "groups" >1rst grp : 1s >2nd grp : 2s2p >3rd grp : 3s3p >4th grp : 3d >5th grp : 4s4p >... > >I think that is not useful to develop further here. > >I was just wondering if Cr{-1} anion exists experimentaly? >I just demonstrate that Slater rules predict it to be more >stable than Cr atom, but I am not very confident with that >type of calculation for transition metal elements. > >Hope this helps. To: "chemistry $#at#$ ccl.net" Subject: CCL:Orbital energies for 1st row transition metals. Sender: "Computational Chemistry List" > I would say that this example has a low > pedagogical value for a first year course except if you want to show the > limitations of Koopman's theorem. Hi all- I wasn't aware that first year chemistry students were being taught about Koopman's theorem. I have taught several first year chem classes and never seen it mentioned once. In fact, my own exposure to it didn't occur until I was a graduate student.