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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: AdiDex on November 06, 2015, 04:02:58 AM

Title: Evidence of ions
Post by: AdiDex on November 06, 2015, 04:02:58 AM
I was reading Inorganic chemistry : Principles of structure and reactivity (By James E. Huheey ) . I found this paragraph in it.

We have no absolute proof of the existence of ions in solid Sodium Chloride . For example thogh our best example that ions are found when it is melted or dissolved in water does not prove that they existed in the solid crystal.

My question is that " what is the present scenario of this problem ?? " . Have we found something new ???
Title: Re: Evidence of ions
Post by: Hunter2 on November 06, 2015, 04:35:27 AM
No I don't think so. What he is saying means we get ions if we melt or dissolve a salt. That does not automatically mean the Crystal contains already the ions. They can be produced during melting or dissolving. But if this is not the case has to be proven with other methods on the crystal salt.
Title: Re: Evidence of ions
Post by: Arkcon on November 06, 2015, 07:13:36 AM
What are some of the methods used to study solids? What evidence do they give us to support or deny ions ina crystalline solid?  These sorts of questions can help you understand what you're really asking.  Although the absolute reality can become metaphysical rapidly.
Title: Re: Evidence of ions
Post by: kriggy on November 06, 2015, 08:47:17 AM
Cant you distinguish between ions and covalently bonded atoms by Xray difraction?
Title: Re: Evidence of ions
Post by: AdiDex on November 06, 2015, 10:18:58 AM
No I don't think so. What he is saying means we get ions if we melt or dissolve a salt. That does not automatically mean the Crystal contains already the ions. They can be produced during melting or dissolving. But if this is not the case has to be proven with other methods on the crystal salt.
yep I agree. By the way i was asking the same .
Title: Re: Evidence of ions
Post by: AdiDex on November 06, 2015, 10:27:16 AM
Cant you distinguish between ions and covalently bonded atoms by Xray difraction?
As fas as I know X-Ray Diffraction is used to determine the interatomic distance . Can you please elaborate your view ?? I have not read X-ray diffraction in so much detail .
Title: Re: Evidence of ions
Post by: kriggy on November 06, 2015, 11:19:33 AM
Yes it is used to determine interatomic distances, position of atoms and can determine types of atoms too. I would guess that if there is electron missing from the atom it would provide different difraction patter and thus it could be identified by xray. I have, sadly, only very basic knowledge of this technique so my post was more question whether it is possible rather than suggestion of technique
Title: Re: Evidence of ions
Post by: Borek on November 06, 2015, 12:52:15 PM
Some of the methods used in crystallography allow measuring of the electron density inside the crystal (they create a density map). That's as close as you can get to finding ions in solid.
Title: Re: Evidence of ions
Post by: AdiDex on November 08, 2015, 12:06:22 AM
Some of the methods used in crystallography allow measuring of the electron density inside the crystal (they create a density map). That's as close as you can get to finding ions in solid.

Can you tell me the name of That method ?? do you know when it was invented ??

Now I am wondering why James E. Huheey think that there is no absolute proof ?? As I have one more book which is quite old (1970's book ) , It also have X-Rays  and Electron Density .
Title: Re: Evidence of ions
Post by: Irlanur on November 08, 2015, 04:18:37 AM
I mean you can also generalize and ask "are there atoms in molecules" ?
Title: Re: Evidence of ions
Post by: AdiDex on November 08, 2015, 04:29:18 AM
I mean you can also generalize and ask "are there atoms in molecules" ?

I think there should be atoms , How can you define molecule without atom ?? I think here is the question about their bonding ...!!
Title: Re: Evidence of ions
Post by: Borek on November 08, 2015, 05:55:06 AM
http://lmgtfy.com/?q=determining+electron+density+in+crystals
Title: Re: Evidence of ions
Post by: Enthalpy on November 09, 2015, 09:33:31 AM
And how should we tell whether Na and Cl are ionized in solid NaCl? Is this question any answerable?

Since every Na is surrounded by Cl atoms that "touch" it, and the electron shells are fuzzy, I don't quite see how we should tell that an electron belongs to Cl rather than Na. It's the same locations except at a few small positions.

But it does make more sense in BaTiO3 and SrTiO3, where Ba and Sr are not completely surrounded but have an eccentric position - or rather, two possible positions, which make these crystals piezoelectric, ferroelectric, and at the proper BaxSr1-x composition versus the temperature, gives them a huge permittivity. For these compounds, Ba and Sr do carry a (partial) charge, whose effect is observable by the strong polarization of the solid.

BaTiO3 and SrTiO3 being strongly nonlinear (they make the type II ceramic capacitors for electronics design) one could observe the saturation polarization, and from the atom movement, infer the partial charge. I stilll haven't done it and would be interested if someone did.

Same for less symmetric crystals like quartz which is piezoelectric. There we can tell that the bond is polarized.

And what about water, with its permittivity of 80? Its polarization results from molecule orientation, for which accurate models exist. Obviously H and O carry a partial charge. Should we call water an ionic compound because of this? No. It's better to say that bonds are more or less polarized.
Title: Re: Evidence of ions
Post by: AdiDex on November 17, 2015, 02:32:17 AM
And what about water, with its permittivity of 80? Its polarization results from molecule orientation, for which accurate models exist. Obviously H and O carry a partial charge. Should we call water an ionic compound because of this? No. It's better to say that bonds are more or less polarized.

I think that there is no sharp boundary between an Ionic Bond And a Non-Ionic Bond but that does not means we should ignore this classification. On broad level there is a clear distinction . The Polarization in the bond of NaCl is quite large(It means that Electron Probability Density is quite large in the neighborhood of Cl )  so it should be defined as Ionic Bond .
Title: Re: Evidence of ions
Post by: AdiDex on November 17, 2015, 02:50:41 AM
Since every Na is surrounded by Cl atoms that "touch" it, and the electron shells are fuzzy, I don't quite see how we should tell that an electron belongs to Cl rather than Na. It's the same locations except at a few small positions.

Whenever there is interaction between 2 waves , they looses their individuality  . So there is no question about the belonging of that electron to the Na or Cl . It was the question about Electron Probability Density .

And I think This question is answerable if it is in the context of the Electron Probability  Density .
Title: Re: Evidence of ions
Post by: Enthalpy on November 17, 2015, 03:37:14 PM
And since the orbitals of Na and Cl are at the same place, how should you tell whether an electron belongs to one or the other? "In the neighbourhood of Cl" is also the neighbourhood of the 6 surrounding Na. And then, how to tell whether the bond is polarized?
Title: Re: Evidence of ions
Post by: Corribus on November 17, 2015, 07:45:50 PM
Cant you distinguish between ions and covalently bonded atoms by Xray difraction?
As fas as I know X-Ray Diffraction is used to determine the interatomic distance . Can you please elaborate your view ?? I have not read X-ray diffraction in so much detail .
I think he's thinking of X-ray photoelectron spectroscopy, which is routinely used to determine the oxidation states and chemical environments of atoms/ions. I don't see why it couldn't be used (in principle) to determine whether the atoms in NaCl are ionized or not.

https://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy

Maybe there's not something I'm missing, though. I don't have a whole lot of experience with this technique.
Title: Re: Evidence of ions
Post by: AdiDex on November 18, 2015, 01:38:52 AM
And since the orbitals of Na and Cl are at the same place, how should you tell whether an electron belongs to one or the other? "In the neighbourhood of Cl" is also the neighbourhood of the 6 surrounding Na. And then, how to tell whether the bond is polarized?

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.nature.com%2Fncomms%2F2014%2F140428%2Fncomms4525%2Fimages%2Fncomms4525-f4.jpg&hash=5757ce9752133a84f7a0d4adaaabaf70)

May be there is some confusion , I said that when two waves interact with each other (overlap with each other) ,they looses their individuality . Same thing happes in the case of NaCl bond , when their electrons overlap with each other , they are no more bound to either Na or Cl , actually They are bound to both of them (As far as I know , I can be wrong , please correct me If I go wrong) .
Then I said my question is quite answerable if we consider the electron probability Density . Of-course It can be measured .
Initially i posted this  because my sir told me that Principle of Structure and Reactivity by James E Huheey is one of the most Reliable  book in Chemistry .

I was uncertain about the meaning of this paragraph . Is there any deep reasoning behind this ? Which I am not able to understand ??
"We have no absolute proof of the existence of ions in solid Sodium Chloride . For example though our best observation is that ions are found when it is melted or dissolved in water does not prove that they existed in the solid crystal."
Title: Re: Evidence of ions
Post by: Irlanur on December 29, 2015, 07:45:50 AM
Quote
I think there should be atoms , How can you define molecule without atom ?? I think here is the question about their bonding ...!!

You can easily define molecular structure with the positions of the nuclei and some electron density. But who is going to tell you were atom x ends and the next one starts?

You can even go one step further. If we don't use the Born-Oppenheimer approximation of fixed nuclei, we don't even know what we mean by "chemical structure".
Title: Re: Evidence of ions
Post by: AdiDex on January 02, 2016, 09:39:41 AM
Quote
we don't even know what we mean by "chemical structure".

What do you meant by this ?? I didn't understand . Chemical structure ...!! Please elaborate it .