| Topic: |
Science > Physics |
| User: |
"Angelo" |
| Date: |
03 Oct 2005 03:46:14 AM |
| Object: |
H^(2-) |
I tried to ask if H^2- could exist in a national physics NG.
I was told that it could'dt exist, even after I suggested that the
second elettron should have been located in the second level
(2s; 2p). Now, independently on the intrinsic stability,
I was told that PEP would forbid such configuration,
which I don't understand why.
Any clarification is welcome.
Thanks very much for your comments,
Angelo
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| User: "Uncle Al" |
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| Title: Re: H^(2-) |
03 Oct 2005 01:18:44 PM |
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Angelo wrote:
I tried to ask if H^2- could exist in a national physics NG.
I was told that it could'dt exist, even after I suggested that the
second elettron should have been located in the second level
(2s; 2p). Now, independently on the intrinsic stability,
I was told that PEP would forbid such configuration,
which I don't understand why.
Any clarification is welcome.
Thanks very much for your comments,
Angelo
Look at the molecular orbitals involved. Does the next electron go
into an anti-bonding orbital? When you calculate the IR and Raman
frequencies, do they all come out real (a true minimum) or are there
imaginary frequencies (a saddlepoint - no stable molecule)?
(H2)- has a net bond order of 1/2 on first inspection. That's not so
bad. Are there deeper subtleties?
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf
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| User: "" |
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| Title: Re: H^(2-) |
03 Oct 2005 01:38:34 PM |
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In article <43417604.94770986@hate.spam.net>, Uncle Al <UncleAl0@hate.spam.net> writes:
Angelo wrote:
I tried to ask if H^2- could exist in a national physics NG.
I was told that it could'dt exist, even after I suggested that the
second elettron should have been located in the second level
(2s; 2p). Now, independently on the intrinsic stability,
I was told that PEP would forbid such configuration,
which I don't understand why.
Any clarification is welcome.
Thanks very much for your comments,
Angelo
Look at the molecular orbitals involved. Does the next electron go
into an anti-bonding orbital? When you calculate the IR and Raman
frequencies, do they all come out real (a true minimum) or are there
imaginary frequencies (a saddlepoint - no stable molecule)?
(H2)- has a net bond order of 1/2 on first inspection. That's not so
bad. Are there deeper subtleties?
Well, I don't know. H2- (as well as more exotic stuff like H3-,
H4-,... etc. is certainly produced in ion sources and, if not quite
stable, is at least meta stable (enough so to survive a travel down
the beamline). But then, there is lots of weird stuff produced in ion
sources. HeH+ comes to mind.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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| User: "Craig" |
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| Title: Re: H^(2-) |
03 Oct 2005 03:46:31 PM |
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Angelo wrote:
I tried to ask if H^2- could exist in a national physics NG.
I was told that it could'dt exist, even after I suggested that the
second elettron should have been located in the second level
(2s; 2p). Now, independently on the intrinsic stability,
I was told that PEP would forbid such configuration,
which I don't understand why.
Any clarification is welcome.
If I understand you correctly, you want a doubly negative hydrogen ion,
one proton with three electrons. I'm pretty confident this will never
be stable, but the Pauli Exclusion Principle has nothing to do with it.
Most elements have a positive electron affinity. That is, if you take
a bare atom in a vacuum and drop an electron onto it, it will form a
singly charged anion and release excess energy. There are a few
exceptions, but let's not even worry about those for the moment. Let's
suppose we can somehow make H- in vacuum, without worrying about
bothersome details like, when H- forms with excess energy, how that
energy gets dissipated.
To make the double anion, you need to drop another electron onto H-.
Electrostatics already objects - you are trying to bring together two
negative charges! To my recollection, there are no elements which are
stable as di-anions in vacuum, even oxygen.
Let's go back to the irregularities in electron affinity. It is found
that the alkaline earths (Be,Mg,Ca, etc.) and the noble gases both have
negative electron affinities. That is, they do not form stable
negative ions. The obvious explanation is that these atoms already
have filled shells (or subshells). The next electron would have to go
in the next higher (sub)shell, with a markedly lower binding energy.
The binding energy is so much lower that, experimentally, it just isn't
stable. Now, to make doubly negative hydrogen, you would need to add
another electron on top of a [He] electron configuration, but with one
less proton in the nucleus. If regular helium won't hold an extra
electron, a system with even less positive charge in the nucleus surely
won't!
Perhaps such a system could be made to exist briefly by someone clever.
I have seen tables of electron affinities, for example:
http://en.wikipedia.org/wiki/Electron_affinity
that give quantitative values for species that are not stable. I don't
know how these numbers are measured. Perhaps the unstable species is
prepared transiently. In any event, a doubly negative hydrogen ion is
not going to be stable.
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| User: "Angelo" |
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| Title: Re: H^(2-) |
04 Oct 2005 06:44:55 AM |
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Craig wrote:
Angelo wrote:
I tried to ask if H^2- could exist in a national physics NG.
I was told that it could'dt exist, even after I suggested that the
second elettron should have been located in the second level
(2s; 2p). Now, independently on the intrinsic stability,
I was told that PEP would forbid such configuration,
which I don't understand why.
Any clarification is welcome.
I'm very, very sorry and owe apologies to you and
all other responders. I posted (hurrily) before leaving.
So, with the misspellings and bad wording I think I've
caused to you all a lot of lost time :-((.
Now that I'm here again and reread what I wrote and
all your postings I realized all this. Hope you all
would like to forgive me, if possible.
If I understand you correctly, you want a doubly negative hydrogen ion,
one proton with three electrons. I'm pretty confident this will never
be stable, but the Pauli Exclusion Principle has nothing to do with it.
Thanks Craig, this I wanted to know, because (as said)
in another local NG (similar to sci.physics) I asked
why the PEP should forbid the electron configuration
1s2; 2s1, for example. I didn't understand the reply,
even if it was in italian, my native language, by a
person I thought a physicist. So I asked him again to
possibly clarify what he meant, and specifically, why
the PEP is involved in his opinion. The second reply
by him (with a more obscure and convoluted speech)
restated that, besides the coulombic repulsion, the
PEP should be violated in that case.
So I decided to ask here.
Now, what you say below, as well as the others' explanations
is quite familiar to me (I teach 'General and Inorganic
Chemistry') in an italian university, but hope that it may
be of help for other readers, at least.
Most elements have a positive electron affinity. That is, if you take
a bare atom in a vacuum and drop an electron onto it, it will form a
singly charged anion and release excess energy. There are a few
exceptions, but let's not even worry about those for the moment. Let's
suppose we can somehow make H- in vacuum, without worrying about
bothersome details like, when H- forms with excess energy, how that
energy gets dissipated.
To make the double anion, you need to drop another electron onto H-.
Electrostatics already objects - you are trying to bring together two
negative charges! To my recollection, there are no elements which are
stable as di-anions in vacuum, even oxygen.
Let's go back to the irregularities in electron affinity. It is found
that the alkaline earths (Be,Mg,Ca, etc.) and the noble gases both have
negative electron affinities. That is, they do not form stable
negative ions. The obvious explanation is that these atoms already
have filled shells (or subshells). The next electron would have to go
in the next higher (sub)shell, with a markedly lower binding energy.
The binding energy is so much lower that, experimentally, it just isn't
stable. Now, to make doubly negative hydrogen, you would need to add
another electron on top of a [He] electron configuration, but with one
less proton in the nucleus. If regular helium won't hold an extra
electron, a system with even less positive charge in the nucleus surely
won't!
Perhaps such a system could be made to exist briefly by someone clever.
I have seen tables of electron affinities, for example:
http://en.wikipedia.org/wiki/Electron_affinity
that give quantitative values for species that are not stable. I don't
know how these numbers are measured. Perhaps the unstable species is
prepared transiently. In any event, a doubly negative hydrogen ion is
not going to be stable.
Your presentation of the topic is excellent!
Thanks again to all,
Angelo
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| User: "tadchem" |
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| Title: Re: H^(2-) |
03 Oct 2005 01:34:31 PM |
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Your notation is not clear.
Are you referring to a single hydrogen atom with a double negative
charge (i.e. one H with *three* electrons*), or a 2-hydrogen molecular
ion with a single negative charge (i.e. two H's with *three*
electrons)?
Tom Davidson
Richmond, VA
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| User: "Angelo" |
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| Title: Re: H^(2-) |
04 Oct 2005 07:00:56 AM |
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tadchem wrote:
Your notation is not clear.
Correct.
Are you referring to a single hydrogen atom with a double negative
charge (i.e. one H with *three* electrons*), or a 2-hydrogen molecular
ion with a single negative charge (i.e. two H's with *three*
electrons)?
The first case, but please, refer to my last post in reply to Craig,
in order to not repeat the reasons and circumstances that
may have brought about such confusion. Sorry Tom.
Tom Davidson
Richmond, VA
Sorry Tom,
Angelo
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