| Topic: |
Science > Physics |
| User: |
"Phil Gardner" |
| Date: |
10 Sep 2003 02:53:32 AM |
| Object: |
Negative ions |
Nearly all atoms can form stable negative ions. At least in principle
any one of these can bond ionically to any positive ion including
those of the same species of atom.
How then do physical chemists and/or chemical physicists decide
whether the bonds that hold such molecules as H2 and N2 together are
covalent or ionic?
Phil Gardner
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| User: "Marvin Margoshes" |
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| Title: Re: Negative ions |
10 Sep 2003 12:31:27 PM |
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"Phil Gardner" <pej_dg@dodo.com.au> wrote in message
news:ea961a86.0309092353.8a8c9a9@posting.google.com...
Nearly all atoms can form stable negative ions.
Can you substantiate that statement? It is news to me.
At least in principle
any one of these can bond ionically to any positive ion including
those of the same species of atom.
How then do physical chemists and/or chemical physicists decide
whether the bonds that hold such molecules as H2 and N2 together are
covalent or ionic?
Phil Gardner
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| User: "Joćo Antonio" |
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| Title: Re: Negative ions |
10 Sep 2003 11:10:40 PM |
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Nearly all atoms can form stable negative ions.
Can you substantiate that statement? It is news to me.
He's talking about electron affinity. The energy released when
one electron is added to a neutral atom in gas phase.
Only for a few elements the energy must be *supplied* for
this to happen. If I'm not mistaken, they are Nitrogen and Beryllium
(cannot find a suitable table right now), it is linked to electron
distribution, electron-electron repulsions and half-filled shells,
this kind of stuff.
Even the ceside anion Cs- can be form. It is stable in solution!!!
IIRC, adding a crown ether to a cesium dispersion in ether it will
form [Cs(crown ether)]+ Cs-.
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| User: "Marvin Margoshes" |
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| Title: Re: Negative ions |
11 Sep 2003 07:48:55 AM |
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"Joćo Antonio" <jas_bomfim@uol.com.br> wrote in message
news:bjotbk$r6v$1@news.mc.ntu.edu.tw...
Nearly all atoms can form stable negative ions.
Can you substantiate that statement? It is news to me.
He's talking about electron affinity. The energy released when
one electron is added to a neutral atom in gas phase.
Only for a few elements the energy must be *supplied* for
this to happen. If I'm not mistaken, they are Nitrogen and Beryllium
(cannot find a suitable table right now), it is linked to electron
distribution, electron-electron repulsions and half-filled shells,
this kind of stuff.
Even the ceside anion Cs- can be form. It is stable in solution!!!
IIRC, adding a crown ether to a cesium dispersion in ether it will
form [Cs(crown ether)]+ Cs-.
---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.516 / Virus Database: 313 - Release Date: 1/9/2003
Does the complex then form ionic crystals with cations? If so, it is the
answer to the original question, I think. But was the questioner thinking
that ions like Cs- could exist on their own?
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| User: "Terry Wilder" |
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| Title: Re: Negative ions |
13 Sep 2003 11:32:04 PM |
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"Phil Gardner" <pej_dg@dodo.com.au> wrote in message
news:ea961a86.0309092353.8a8c9a9@posting.google.com...
Nearly all atoms can form stable negative ions. At least in principle
any one of these can bond ionically to any positive ion including
those of the same species of atom.
How then do physical chemists and/or chemical physicists decide
whether the bonds that hold such molecules as H2 and N2 together are
covalent or ionic?
Phil Gardner
In reverse order, usually whether they show any electrochemical activity in
solution in dielectric solvents at low voltages.
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| User: "Repeating Decimal" |
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| Title: Re: Negative ions |
14 Sep 2003 11:24:30 AM |
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in article 8bS8b.6555$1D5.4904@nwrddc02.gnilink.net, Terry Wilder at
terry.wilder@gte.net wrote on 9/13/03 9:32 PM:
"Phil Gardner" <pej_dg@dodo.com.au> wrote in message
news:ea961a86.0309092353.8a8c9a9@posting.google.com...
Nearly all atoms can form stable negative ions. At least in principle
any one of these can bond ionically to any positive ion including
those of the same species of atom.
How then do physical chemists and/or chemical physicists decide
whether the bonds that hold such molecules as H2 and N2 together are
covalent or ionic?
Phil Gardner
In reverse order, usually whether they show any electrochemical activity in
solution in dielectric solvents at low voltages.
It should be clear that such compounds cannot be ionic. That would imply an
different character to the constituent atoms. One atom stably adds and
electron while the other, stably looses an electron. There could be weak
binding forces such as by van der Waal [sp?] interaction, but that would be
weak indeed.
Bill
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| User: "abc" |
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| Title: Re: Negative ions |
15 Sep 2003 05:57:55 AM |
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It should be clear that such compounds cannot be ionic. That would imply
an different character to the constituent atoms. One atom stably adds and
electron while the other, stably looses an electron. There could be weak
binding forces such as by van der Waal [sp?] interaction, but that would
be weak indeed.
Bill
Diatomic molecules, like N2 , not. But whether true homoatomic ionic
compounds exist, like N5- N7+, is still an unresolved question.
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| User: "Terry Wilder" |
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| Title: Re: Negative ions |
15 Sep 2003 04:24:36 PM |
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"abc" <ich@mag.kein.spam.com> wrote in message
news:bk460m$ijq$1@wsc10.lrz-muenchen.de...
It should be clear that such compounds cannot be ionic. That would imply
an different character to the constituent atoms. One atom stably adds
and
electron while the other, stably looses an electron. There could be weak
binding forces such as by van der Waal [sp?] interaction, but that would
be weak indeed.
Bill
Diatomic molecules, like N2 , not. But whether true homoatomic ionic
compounds exist, like N5- N7+, is still an unresolved question.
Nor would one necessarily expect a fullerene to give up an electron to a
carbon ion.
The more accurate models of H2 do assume some ionic character.
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| User: "Repeating Decimal" |
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| Title: Re: Negative ions |
15 Sep 2003 01:32:21 PM |
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This message is in MIME format. Since your mail reader does not understand
this format, some or all of this message may not be legible.
--MS_Mac_OE_3146470396_477536_MIME_Part
Content-type: text/plain; charset="US-ASCII"
Content-transfer-encoding: 7bit
in article bk460m$ijq$1@wsc10.lrz-muenchen.de, abc at
wrote on 9/15/03 3:57 AM:
Diatomic molecules, like N2 , not. But whether true homoatomic ionic
compounds exist, like N5- N7+, is still an unresolved question.
I was just answering the question as asked. IIRC the molecules in question
were H2 and N2. If HCl were mentioned, my answer would be different.
Bill
--MS_Mac_OE_3146470396_477536_MIME_Part
Content-type: text/html; charset="US-ASCII"
Content-transfer-encoding: quoted-printable
<HTML>
<HEAD>
<TITLE>Re: Negative ions</TITLE>
</HEAD>
<BODY>
in article bk460m$ijq$1@wsc10.lrz-muenchen.de, abc at =
wrote on 9/15/03 3:57 AM:<BR>
<BR>
> Diatomic molecules, like N2 , not. But whether true homoatomic ionic <=
BR>
> compounds exist, like N5- N7+, is still an unresolved question.<BR>
<BR>
I was just answering the question as asked. IIRC the molecules in question =
were H<FONT SIZE=3D"1">2</FONT> and N<FONT SIZE=3D"1">2</FONT>. If H=
Cl were mentioned, my answer would be different.<BR>
<BR>
Bill
</BODY>
</HTML>
--MS_Mac_OE_3146470396_477536_MIME_Part--
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| User: "Stephan Bird" |
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| Title: Re: Negative ions |
15 Sep 2003 01:52:31 PM |
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In article <bk460m$ijq$1@wsc10.lrz-muenchen.de>,
says...
It should be clear that such compounds cannot be ionic. That would imply
an different character to the constituent atoms. One atom stably adds and
electron while the other, stably looses an electron. There could be weak
binding forces such as by van der Waal [sp?] interaction, but that would
be weak indeed.
Bill
Diatomic molecules, like N2 , not. But whether true homoatomic ionic
compounds exist, like N5- N7+, is still an unresolved question.
Interestingly enough, there's a bit of a discussion in this month's
Chemistry in Britain about the existence (or rather the non-existence,
perhaps) of N5-N5+ and N3-N5+,. See e.g. Angew. Chem. Int. Ed. _1999_,
*38*, 2004, idem., _2002_, *41*, 3051 and a JACS paper ("in press",
haven't seen it yet personally), by Christe, Vij, Wilson, Vij, Dixon,
Feller and Jenkins (not sure about the actual paper authors, these are
just the letter writers...
Stephan
--
Stephan Bird MChem(Hons) AMRSC
Currently in Oxford, England
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| User: "Joćo Antonio" |
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| Title: Re: Negative ions |
15 Sep 2003 08:18:42 PM |
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The plain quick answer is electronegativity and bond polarity.
Vand der Waals has nothing to do with it and electrolitic
behaviour would be an experimental aproach, but even
covalently bonded molecules may ionise in solution generating
solvated ions.
"Repeating Decimal" <salmonfry@sbcglobal.net> escreveu na mensagem
news:BB89E483.A200%salmonfry@sbcglobal.net...
in article 8bS8b.6555$1D5.4904@nwrddc02.gnilink.net, Terry Wilder at
terry.wilder@gte.net wrote on 9/13/03 9:32 PM:
"Phil Gardner" <pej_dg@dodo.com.au> wrote in message
news:ea961a86.0309092353.8a8c9a9@posting.google.com...
Nearly all atoms can form stable negative ions. At least in principle
any one of these can bond ionically to any positive ion including
those of the same species of atom.
How then do physical chemists and/or chemical physicists decide
whether the bonds that hold such molecules as H2 and N2 together are
covalent or ionic?
Phil Gardner
In reverse order, usually whether they show any electrochemical activity
in
solution in dielectric solvents at low voltages.
It should be clear that such compounds cannot be ionic. That would imply
an
different character to the constituent atoms. One atom stably adds and
electron while the other, stably looses an electron. There could be weak
binding forces such as by van der Waal [sp?] interaction, but that would
be
weak indeed.
Bill
---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.518 / Virus Database: 316 - Release Date: 11/9/2003
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| User: "Richard Schultz" |
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| Title: Re: Negative ions |
10 Sep 2003 05:14:44 AM |
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In sci.chem Phil Gardner <pej_dg@dodo.com.au> wrote:
: How then do physical chemists and/or chemical physicists decide
: whether the bonds that hold such molecules as H2 and N2 together are
: covalent or ionic?
The basic "freshman-level" explanation is that the amount of "ionic
character" of a bond depends on the difference in electronegativity
between the two atoms. In a homonuclear diatomic molecule such as H2, there
is no electronegativity difference, so the bond is non-polar by definition.
Remember that an "ionic bond" is *defined* as one in which there is a
*net* separation of charge. Suppose that there were such a bond in an
H2 molecule, i.e., it were H+H-. In that case, we'd be able to tell the
two H atoms apart, which is forbidden -- all H atoms are identical. So the
only way that an "ionic bond" would work in that sense is if it flipped back
and forth between H+H- and H-H+. Even if such a process were occurring,
there would be no *net* separation of charge as there is in, say, NaCl, so
the bond would *look* covalent experimentally.
Note the *if* in the previous sentence -- the situation is analogous to that
of trying to describe benzene as alternating single and double bonds. Since
all of the bonds in benzene are the same length, that description cannot be
correct. *If* the bonds were flipping back and forth too fast for us to
measure, then the molecule would look like all of the bonds were the same
length. That is *not* what is happening, however; rather, these simple
descriptions are simply inadequate to model the molecule.
A more sophisticated way of approaching the question is to start from the
assumption that the bond is perfectly covalent and see if that model
predicts the properties (bond length, bond dissociation energy) of the H2
molecule. That means that we have to treat the system quantum mechanically
and write a wave function that puts one electron on each atom. It turns
out that this description is inadequate, and does *not* accurately
represent the properties of an actual H2 molecule. One way of improving
the situation is to include the ionic wavefunctions for H+H- and H-H+ in
the wavefunction of the molecule, and it turns out that this wavefunction
does much better. (Note for advanced students: I'm ignoring configuration
interaction here.)
So the short answer is that the H2 bond cannot be ionic because two
identical atoms cannot give a bond with permanent net charge separation.
The long answer is that in fact structures with charge separation actually do
"contribute" to the overall structure of the H2 bond, but that since the
overall wavefunction has to keep the same net charge on both atoms, the
bond is covalent.
-----
Richard Schultz
Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel
Opinions expressed are mine alone, and not those of Bar-Ilan University
-----
"Logic is a wreath of pretty flowers which smell bad."
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| User: "abc" |
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| Title: Re: Negative ions |
10 Sep 2003 10:19:22 AM |
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Richard Schultz wrote:
The basic "freshman-level" explanation is that the amount of "ionic
character" of a bond depends on the difference in electronegativity
between the two atoms. In a homonuclear diatomic molecule such as H2,
there is no electronegativity difference, so the bond is non-polar by
definition.
First let me say that you gave a very nice explanation.
I think one can expand that electronegativity stuff further:
According to Pauli, the EN is proportional to the difference of elektron
affinity and ionization potential. If both were equal, in a homonuclear
diatomic molecule, there would be no cost in transfering an electron from
one atom to the other. Therefore, both ionic and covalent structures would
appear with equal weight in the total wavefunction. However, this would
mean, that the Hartree Fock wavefunction would be very good, as the HF
wavefunction, when projected onto covalent and ionic structures, consists
of equal parts of covalent and ionic structures.
This situation is found approximately in metals, for which the EN is small.
It also explains that the wavefunction in bulk metals is very HF like.
The other extreme is formed by the halogens. As the EN is big, the
wavefunction is nearly completely covalent. In case of F_2, HF fails to
predict a bond between the two F atoms, while a valence bond calculation
with just one covalent bond does.
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